Debates in PsychologyThe synoptic content of this chapter focuses on debates that concern psychologists. A debate is discussion of an issue, usually involving the consideration of different sides of a question. As you are already aware, none of these debates is simply “one or the other”—there is nearly always some reasonable middle ground.

Ask yourself Is it possible to take a position of free will and at the same time advocate

scientific research? Is it preferable to reduce explanations to their simplest components? What alternatives are there to reductionism? Why is it mistaken to talk about nature or nurture? How does gene–environment interaction make it difficult to answer the

nature–nurture question? What are the practical implications of the nature–nurture debate?

FREE WILL AND DETERMINISMThe issue of free will versus determinism has occupied philosophers and psychologists for centuries. According to those who believe in determinism, people’s actions are totally determined by the external and internal forces operating on them. An example of an external force would be the influence of parents when rewarding certain behaviours. An example of an internal force could be hormones influencing the way in which someone behaves.

Those who believe in free will argue that matters are more complicated. Most of them accept that external and internal forces exist. However, they argue that people have free will because each individual is nevertheless able to choose his/her own behaviour within the constraints of these forces. Note that the positions adopted by advocates of determinism and of free will are not that far apart— determinists argue that behaviour is totally controlled by external and internal forces, whereas those favouring free will argue that behaviour is mostly controlled by external and internal forces but with the addition of free will.

The distinction between free will and determinism can be seen if we consider the following question: “Could an individual’s behaviour in a given situation have been different if he/she had willed it?” Believers in free will answer that question “Yes”. In contrast, advocates of determinism respond “No”. Some of the main arguments for and against each of these positions are discussed next.

DeterminismDeterminists argue that a proper science of human behaviour is only possible if psychologists adopt a deterministic account, according to which everything that happens has a definite cause. Free will, by definition, doesn’t have a definite cause. If free will is taken into account, it becomes impossible to predict human behaviour with any precision. According to some determinists, it is often possible with other

sciences to make very accurate predictions from a deterministic position (e.g., forecasting planetary motion). If determinism is regarded as not applicable to psychology, then psychology is either a very different science from physics, chemistry, and so on, or it isn’t really a science at all.

At this point, we need to distinguish among different types of determinism. Our behaviour is influenced by numerous factors, and theorists differ in terms of the relative importance they attach to each factor. For example, evolutionary psychologists focus on ultimate or fundamental causes of behaviour based on the evolutionary history of the human species. We could call this evolutionary determinism. In contrast, most psychologists focus on proximate or immediate causes of behaviour based on the external and/or internal forces operating at any given moment. Behaviourists such as Skinner emphasise external forces in the environment (e.g., rewards and punishments), and we could call this environmental determinism. In contrast, Freud emphasised the way in which our behaviour is influenced by internal forces involving conflicts among the ego, id, and superego. He called this psychic determinism. Social psychologists emphasise the importance of social forces (e.g., pressures to conform, obedience to authority) in influencing our behaviour—we could call this social determinism. We could go further in identifying different types of determinism. If we tried to produce a complete list of factors influencing our behaviour, it would contain at least the following factors:

The specific stimuli presented to us Our recent experiences (e.g., being stuck in a traffic jam) Our genetic make-up Our evolutionary history Our physiological system Our cognitive system (e.g., our perceptions, thoughts, and memories) The social environment The cultural environment Our previous life experiences (including those of childhood) Our personal characteristics (including intelligence, personality, and mental

health).

Hard vs. soft determinismHard determinismWe have seen that there are various types of determinism. At a more general level, we can distinguish between hard determinism and soft determinism. Hard determinism as it applies to psychology is based on two key assumptions. First, no action or behaviour is free if it must occur. Second, every human action has antecedent (preceding) causes that ensure that one particular action is performed rather than any other. The conclusion from these assumptions is that all human actions are determined and none of them is free. Those who believe in hard determinism include B.F. Skinner and Sigmund Freud.

Hard determinism has been applied extensively in other sciences (especially

physics). It seemed appropriate in the eighteenth and nineteenth centuries when most physicists believed they would eventually be able to make very precise and accurate predictions about everything relevant to physics. However, what happened in the twentieth century suggested they were unduly optimistic. For example, according to chaos theory (Hilborn, 1994), very small changes in initial conditions can produce major changes later on. For example, theoretically the flap of a butterfly’s wing in one part of the world could ultimately change the whole weather system in a different part of the world. Such a chain of events doesn’t lend itself to prediction, and so we can’t show that an approach based on hard determinism is appropriate. More generally, it isn’t really possible to test directly the assumptions of hard determinism.

Soft determinismMany (probably most) psychologists favour an alternative position labelled soft determinism by William James. According to this position, it is accepted that all human actions have a cause. However, and this is where soft determinists part company with hard determinists, there is a valid distinction between behaviour highly constrained by the situation (involuntary behaviour) and behaviour only modestly constrained by the situation (voluntary behaviour). For example, a child may apologise for swearing because he/she will be punished if an apology isn’t forthcoming (highly constrained behaviour) or because he/she is genuinely upset at causing offence (modestly constrained behaviour). Behaviour is determined in both cases. However, the underlying causes are more obvious when behaviour is highly constrained by situational forces.

Evidence consistent with the views of William James was reported by Westcott (1982). Canadian students indicated how free they felt in various situations. They experienced the greatest feeling of freedom in situations involving an absence of responsibility or release from unpleasant stimulation (e.g., a nagging headache). In contrast, they felt least free in situations in which they had to recognise there were limits on their behaviour (e.g., when they had to curtail their desires to fit their abilities).

Evidence suggesting we don’t always know accurately whether our behaviour is voluntary or involuntary was reported by Wegner and Wheatley (1999). Two participants (one genuine and the other a confederate working for the experimenter) placed their fingers on a small square board. When they moved the board, this caused a cursor to move over a screen showing numerous pictures of small objects. The genuine participants thought they had voluntarily decided which object the cursor would stop on (they had heard the name of the object over their headphones just beforehand). In fact, however, the decision had been taken by the confederate—thus, the participants believed their behaviour to be voluntary but it was actually involuntary.

Ask yourself: How is determinism related to the situation in which a behaviour occurs? Is the behaviour still determined by forces outside our will?

There are various limitations with soft determinism. First, there is excessive reliance on how we subjectively perceive things—the fact that some actions feel voluntary whereas others feel involuntary doesn’t mean they are really different. Second, we sometimes make mistakes when deciding whether our behaviour is voluntary or involuntary (e.g., Wegner & Wheatley, 1999). Third, it can be argued that soft determinists want to have their cake and eat it—actions are free if they are voluntary, but those actions are still caused. This could be regarded as a confusing blend of free will and determinism.

Find out more: Where do the main approaches in psychology stand on determinism?

Behaviourist and Freudian approaches Determinism is espoused by more approaches in psychology than is free will. As was mentioned already, the behaviourists and Freud are among those theorists who argue in favour of hard determinism, and so represent the greatest contrast with the views of those believing in free will. We will consider their views here.

Behaviourism The behaviourists believed especially strongly in determinism. Skinner argued that virtually all of our behaviour is determined by environmental factors. He proposed that we repeat behaviour that is rewarded, and we don’t repeat behaviour that isn’t rewarded. Other behaviourists argued that we can predict how someone will respond given knowledge of the current stimulus situation and that individual’s previous conditioning history.

Skinner (1971) developed his ideas about hard determinism most fully in his book, Beyond Freedom and Dignity. He argued that common beliefs about free will and personal moral responsibility (which he called “dignity”) were wrong and should be abandoned for the sake of improving society. According to Skinner, the way to change human behaviour is by structuring the environment so that people are rewarded for behaving in desirable ways (i.e., operant conditioning) rather than by focusing on meaningless notions like freedom and dignity.

Bandura (1977, p.27) pointed out a serious limitation with Skinner’s approach: “If actions were determined solely by external rewards and punishments, people would behave like weather vanes, constantly shifting in radically different directions to conform to the whims of others.” This criticism applies more forcefully to the human species than to non-human species, because we are much more likely to act in line with long-term goals.

What is missing from Skinner’s approach? Skinner focused excessively on the notion that the external environment determines behaviour. However, our behaviour also determines the external environment—if you don’t like a television programme you are watching, you switch to another channel or turn the television off. In addition,

our personality helps to determine the environment in which we find ourselves and it also influences our behaviour. Thus, there are multiple determinants of behaviour, but Skinner largely ignored most of them.

FreudFreud was also a strong believer in hard determinism, claiming that none of our behaviour “just happens” or is due to free will. He even argued that trivial phenomena, such as missing an appointment, calling someone by the wrong name, or humming a particular tune had definite causes within the individual’s motivational system. Of particular importance is what is known as the Freudian slip—a motivated but involuntary error in which someone says or does something revealing their true desires. Motley et al. (1983) obtained evidence of Freudian slips. Male participants had to say out loud pairs such as tool—kits, some of which could be turned into sexually explicit words. When the experimenter was an attractive female, participants tended to make Freudian slips—for example, saying cool—tits instead of tool—kits. It is difficult to see how we could explain such findings in terms of free will.

Ask yourself: Think of a time when you have called someone by the wrong name. Can you think of any underlying reason why you may have made this mistake?

Freud’s emphasis on determinism and rejection of free will may well owe something to the fact that he focused on individuals suffering from mental disorders (especially anxiety disorders). Such individuals are presumably highly motivated to change their behaviour and eliminate their disorder but are often unable to do so—this seems somewhat difficult to explain if they possess free will. It seems more plausible to assume (as Freud did) that their behaviour is determined by forces they cannot control. Note that one of the distinguishing features of Freud’s deterministic approach was his emphasis on unconscious forces influencing our behaviour—according to him, we are often unaware of what causes us to behave in certain ways.

TestabilityThe major problem with determinism (whether soft or not) is that it is not really possible to submit it to a proper test. If it were, then the issue of free will versus determinism would have been settled, and so would no longer exist as an issue! If all behaviour is determined by internal and external forces, then in principle it should be possible to predict behaviour from a knowledge of these causal factors. In fact, we usually only have very limited knowledge of the internal and external forces that might be influencing an individual’s behaviour. As a result, it remains no more than an article of faith that human behaviour can eventually be predicted accurately.

Free WillMost people feel that they possess free will, in the sense that they can freely choose what to do from a number of options. As Dr Samuel Johnson (1709–1784) said to Boswell, “We know our will is free, and there’s an end on’t.” Most people also have

feelings of personal responsibility, presumably because they feel that they are in at least partial control of their behaviour. Another argument in favour of free will is that it fits with society’s view that people should accept responsibility for their actions and should expect to be punished (e.g., sent to prison) if they break the law.

Ask yourself: How might the notions of free will and determinism be important in a situation where doctors need to decide if a criminal is responsible for his or her own actions?

Humanistic approachHumanistic psychologists such as Carl Rogers and Abraham Maslow are among those who believe in free will. They argued that people exercise choice in their behaviour, and they denied that people’s behaviour is at the mercy of outside forces. Rogers’ client-centred therapy is based on the assumption that the client has free will. The therapist is called a “facilitator” precisely because his/her role is to make it easier for the client to exercise free will in such a way as to maximise the rewardingness (reward potential) of the client’s life. Humanistic psychologists argue that regarding human behaviour as being determined by external forces is “de-humanising” and incorrect.

Rogers emphasised the notion that we are motivated to minimise the discrepancy between our self-concept and our ideal self (the self-concept we would most like to possess). If we have free will and our behaviour isn’t determined by external forces, it might be expected that we would have little difficulty in reducing any discrepancy between our self-concept and ideal self. The fact that there are millions of people with mental disorders who have a substantial discrepancy between the two suggests that free will is often very ineffective in producing highly desired changes or even that it doesn’t exist.

CausalityThose who believe in free will have to confront various problems. First, it is hard to provide a precise account of what is meant by free will. If free will has a major influence on human behaviour, it is very important that we define free will precisely. Second, determinism is based on the assumption that all behaviour has one or more causes, and it could be argued that free will implies that behaviour is random and has no cause. However, very few people would want to argue for such an extreme position. Anyone whose behaviour seemed to be random would probably be classified as mentally ill or very stupid! If free will doesn’t imply that behaviour has no cause, then we need to know how free will plays a part in causing behaviour.

Third, most sciences are based on the assumption of determinism. It is possible that determinism applies to the natural world but doesn’t apply to humans. If that is the case, then there are enormous implications for psychology that have hardly been addressed as yet.

Determinism vs. free will Determinism Free will BehaviourismFreudian psychodynamics

Humanistic approach

Ask yourself: Do you think the cognitive psychologists fit into one or other of these lists? Can you explain your answer?

Conclusions The issue of free will versus determinism has created more heat than light for various reasons. First, it isn’t clear that it makes much sense to talk about “free will”, because this assumes there is an agent (i.e., the will) that may or may not operate in an unrestrained way. As the philosopher John Locke (1632–1704) pointed out, “We may as properly say that the singing faculty sings and the dancing faculty dances as that the will chooses” (1690/1989).

Second, the issue is philosophical rather than scientific, as it is impossible to design an experiment to decide whether or not free will influences human behaviour. As William James (1890, p.323) put it, “the fact is that the question of free will is insoluble on strictly psychological grounds”. Thus, we can never know whether an individual’s behaviour in a given situation could have been different if he/she had so willed it.

Third, although those who believe in determinism or free will often seem to have radically different views, there is more common ground between them than generally realised. Regardless of their position on the issue of free will versus determinism, most psychologists accept that heredity, past experience, and the present environment, all influence our behaviour. Although some of these factors (such as the environment) are external to the individual, others are internal. Most of these internal factors (such as character or personality) are the results of causal sequences stretching back into the past. The dispute then narrows to the issue of whether a solitary internal factor (variously called free will or self) is somehow immune from the influence of the past.

Fourth, and most important, we can go a step further and argue that there is no real incompatibility between determinism and free will at all. According to determinists, it is possible in principle to show that an individual’s actions are caused by a sequence of physical activities in the brain. If free will (e.g., conscious thinking and decision making) forms part of that sequence, it is possible to believe in free will and human responsibility at the same time as holding to a deterministic position. This would not be the case if free will is regarded as an intruder forcing its way into the sequence of physical activities in the brain, but there are no good grounds for adopting this position. Thus, the entire controversy between determinism and free will may be artificial and of less concern to psychologists than has generally been supposed.

The issue of free will versus determinism was considered in detail by Valentine (1992). In spite of the various criticisms of the deterministic position, she came to the following conclusion: “Determinism seems to have the edge in this difficult debate.”

Activity: Free will and decisions

SECTION SUMMARY: Free Will and DeterminismThe difference between free will and determinism

One way to consider the debate of determinism versus free will is to ask the following question:

o "Could an individual’s behaviour in a given situation have been different if he/she had willed it?"

Determinists argue that all human behaviour has a definite cause. The scientific approach is a deterministic one and it used to be thought that, if we allow for free will, then psychology isn’t really a science at all. However, even in the physical sciences uncertainty and chaos are now recognised principles.

More psychologists believe in determinism than in free will. The major problem with determinism is that it isn’t possible to submit it to a

proper experimental test.Different types of determinism

Evolutionary determinism: fundamental causes of behaviour are based on the evolutionary history of the human species.

Environmental determinism: behaviour is caused by external forces in the environment (e.g., rewards and punishments).

Psychic determinism: behaviour is influenced by internal forces involving the ego, id, and superego.

Social determinism: behaviour is influences by social forces (e.g., conformity, obedience to authority).

Hard vs. soft determinism Hard determinism:

o According to those who favour hard determinism, all human actions are determined and none is free.

o Skinner and Freud are two psychologists who believed in hard determinism.

Soft determinism: According to those who advocate soft determinism, some behaviour is highly

constrained by the situation (involuntary behaviour), whereas other behaviour is not (voluntary behaviour).

This permits some adaptability in the definition of determinism. Many psychologists believe in soft determinism.

Behaviourist and Freudian approaches Skinner argued that our behaviour was determined by environmental

factors. This approach can be criticised:

o Bandura argued that humans would not be constant.o Skinner ignored other determinants of behaviour.

Freud emphasised determinism and rejected free will.Free will

Most people feel they possess free will, because they feel able to choose freely what to do in most situations.

Humanistic psychologists believe in free will, which is the basis for client-centred therapy.

The belief in free will creates two problems:o It is hard to provide a precise account of what is meant by free will.o Most successful sciences are based on the assumption of determinism.

Conclusions The debate is largely a philosophical one because it cannot be subjected to

testing. Most psychologists accept that heredity, past experience, and the present environment all influence behaviour, and so the key issue is whether there is an internal factor (free will) that also influences behaviour.

The debate may simply be artificial; if free will forms part of physical activities of the brain, it is possible to believe in free will at the same time as holding a deterministic position.

REDUCTIONISMAccording to Reber (1995), reductionism is “the philosophical point of view that complex phenomena are best understood by a componential analysis which breaks the phenomena down into their fundamental, elementary aspects”, or “the analysis of complex things into simple constituents” (Oxford Concise Dictionary). Within the context of psychology, the term refers to several rather different theoretical approaches. First, there is the belief that the phenomena of psychology can potentially be accounted for within the framework of more basic sciences or disciplines (physiological reductionism). Second, there is the assumption that complex forms of behaviour can be explored by simple experiments (experimental reductionism). Third, there is the notion that the complexities of human cognition can be compared to computer functioning (machine reductionism). Fourth, there is the assumption that human behaviour can be understood with reference to other, less complex species (animal reductionism). Fifth, there is the assumption that human behaviour can be understood in terms of genetic factors (evolutionary reductionism). We will consider each of these forms of reductionism in turn.

Ask yourself: Can you think of some issues within psychology that might lend themselves to a reductionist approach?

Physiological ReductionismAccording to physiological reductionism, we need to consider psychology in the light of other scientific disciplines. Of particular importance, all psychological processes are accompanied by physiological processes, and understanding those physiological processes might assist us in understanding human behaviour. Our behaviour is also affected by various social processes (e.g., conformity, the desire to impress others).

Scientific disciplines can be regarded as being organised in a hierarchical way, with the sciences adopting a more global perspective at the top, and the more narrowly focused sciences at the bottom:

Sociology: the science of groups and societies Psychology: the science of human and animal behaviour Physiology: the science of the functional working of the healthy body Biochemistry: the science of the chemistry of the living organism.

Reductionists argue that the sciences towards the top of the hierarchy will eventually be replaced by those towards the bottom—if so, it should ultimately be possible to explain psychological phenomena in physiological or biochemical terms.

Find out more: Physiological and psychological explanations

Physiological reductionism has some advantages. First, it has an immediate appeal. Biochemistry, physiology, psychology, and sociology are all concerned with human functioning, so there is some overlap in their subject matter. This is especially the case with brain imaging, which provides detailed information about activity in different parts of the brain when we are performing some task. There might be an increased understanding of psychology resulting from taking full account of the relevant contributions of other sciences.

Second, biochemistry and physiology can be regarded as more developed than psychology or sociology, since they have more well-established facts and theories.

Third, even those unconvinced of the benefits of reductionism generally accept that psychological theories should be consistent or compatible with physiological findings. For example, the findings from brain-imaging research are relevant to cognitive psychologists trying to understand human cognition.

Physiological reductionism has various disadvantages. First, much human behaviour cannot be understood solely in terms of basic biological and physiological processes. Suppose a psychologist wants to predict how a group of people will vote in a forthcoming election. No-one in their right mind would argue that a detailed biochemical and physiological examination of their brains would be of much value! Most psychologists would argue that we need all levels of explanation to provide an adequate account of human behaviour.

Second, as Valentine (1992) pointed out, psychology typically describes the processes involved in performing some activity (e.g., visual perception), whereas physiology focuses more on the structures involved. Thus, psychologists are interested in how questions, whereas physiologists are interested in where questions.

Third, physiological reductionism hasn’t worked very well in practice—very few psychological phenomena have been explained in physiological or biochemical terms.

Fourth, reductionist explanations (such as those provided by physiologists) often contain many irrelevant details from the perspective of psychology. This can make it very hard to identify what is relevant and in a physiological account.

Experimental ReductionismMost of experimental psychology is based on the assumption that the complexities of human behaviour can be studied effectively in relatively simple experiments—this is known as experimental reductionism. The behaviourists were firmly in favour of experimental reductionism. For example, Skinner carried out studies on rats in which they were placed on their own in so-called “Skinner boxes” that contained little other than a lever to press.

The first advantage of experimental reductionism is that use of the experimental method provides the most scientific (and replicable) evidence we have concerning human behaviour. If we tried to study all the factors influencing human behaviour at the same time, the task would be impossibly complex and the findings uninterpretable. Second, experimental reductionism is the obvious approach to take from the perspective of some theories. For example, the behaviourists argued that learning involves making stimulus–response associations, and experiments involve presenting stimuli and observing responses.

A disadvantage with experimental reductionism is that it omits much that is very important in determining human behaviour. Wachtel (1973) coined the term implacable experimenter to describe the typical experimental situation in which the behaviour of the experimenter occurs independently of the participant’s behaviour. Since the situation is allowed to influence the participant but the participant isn’t allowed to influence the situation, it is likely that the effects of situations on our behaviour are exaggerated. In addition, this approach doesn’t permit any of the dynamic interactions between individuals we experience much of the time. A second disadvantage is that the emphasis within experimental reductionism is on factors in the immediate situation manipulated by the experimenter. Our behaviour is also influenced by genetic factors, by our present mood, by our childhood experiences, by our present state of health, and so on, but these factors are typically ignored in psychological experiments.

Machine ReductionismThe essence of machine reductionism is that we can understand human thinking and/or behaviour by comparing ourselves with various kinds of machines. An influential form of machine reductionism is found within cognitive psychology— some cognitive psychologists believe there are important similarities between human cognition and computer functioning. For example, computers engage in information processing, and they can store information either briefly (as in human short-term memory) or for long periods of time (as in human long-term memory).

Machine reductionism involving computers has various advantages. First, we can compare the functioning of humans and computers to see how similar they are. If computers don’t function like humans, we can reprogramme the computer to improve the similarity between them. For example, the human brain is strongly interconnected, and psychologists have devised computer-based neural or connectionist networks that possess that quality.

A clear disadvantage of machine reductionism is that it omits many factors determining human behaviour. For example, human cognition and behaviour are often strongly influenced by motivational and emotional factors, but a computer’s functioning is not influenced by its mood or goals! Another disadvantage is that the human brain is incredibly complex and machine reductionism involves a grossly oversimplified view of its complexity. For example, the number of units in computer neural or connectionist networks is a tiny fraction of the cells in the brain.

Animal ReductionismThe essence of animal reductionism is the assumption that we can understand human behaviour by studying the behaviour of other species regarded as less complex than the human one. This approach is especially associated with the behaviourists. For example, Pavlov carried out studies on dogs and Skinner on rats and pigeons, but they assumed that their findings were fully applicable to humans. Another behaviourist (Clark Hull) argued that all species learn in the same way, but humans typically learn faster and more efficiently than other species.

There are various advantages of animal reductionism. First, it can be argued that simple forms of learning (e.g., classical conditioning, operant conditioning) can be studied more directly in other species than in the human one. Our behaviour is influenced by language, by social factors, and by cultural factors, and these may complicate the interpretation of findings. Second, some phenomena initially observed in other species have important implications for humans. For example, Seligman (1975) discovered learned helplessness in dogs, and this led to an enhanced understanding of depression in humans (see Eysenck’s A2 Level Psychology Chapter 11, Psychopathology: Depression).

What are the disadvantages of animal reductionism? First, the human species is so different from other species that this greatly limits what we learn by studying them. For example, our large neocortex relative to other species equips us with cognitive

abilities (e.g., language) that simply don’t exist in other species (see A2 Level Psychology Chapter 8, Intelligence and Learning). Second, the findings obtained from studies based on animal reductionism can be very misleading so far as the human species is concerned. Skinner’s research on rats revealed that they are motivated to obtain immediate rewards and avoid immediate punishments. However, such research tells us precisely nothing about important aspects of human motivation such as the way our behaviour is influenced by long-term goals (e.g., achieving a good grade in A-level Psychology).

Evolutionary ReductionismAccording to advocates of evolutionary reductionism, much of the behaviour of the human species can be understood in terms of natural selection. As a result of natural selection, most of our behaviour is adaptive, although there are exceptions because of genome lag. Genetic factors are regarded as being of key importance within evolutionary reductionism because natural selection operates on the basis of certain genes or combinations of genes being favoured by any given environment.

One advantage of evolutionary reductionism is that it encourages us to consider genetic factors when trying to understand human behaviour. There are several areas of psychology in which this has proved useful—examples include individual differences in intelligence and factors causing mental disorders such as schizophrenia and bipolar depression. Second, evolutionary psychology has much to contribute to an understanding of differences among species.

The greatest disadvantage with evolutionary reductionism is that it is limited in scope. Natural selection and genetic factors are important, but so are numerous environmental, social, and cultural factors. Another disadvantage is that even when genetic factors are important, they are nearly always only important in interaction with other factors not considered within evolutionary reductionism. For example, someone who inherits the genes for high intelligence will only realise his/her potential provided there is a supportive environment.

Find out more: Where do the main approaches in psychology stand on reductionism?

Summary of the Advantages and Disadvantages of ReductionismAll forms of reductionism serve the useful function of encouraging us to consider factors that are relevant to an understanding of human behaviour. However, note that these forms of reductionism are not detailed theories in the sense of producing testable hypotheses. What they do is to provide a set of assumptions that can be used to guide theory and research. The various forms of reductionism taken together provide a reasonable basis for understanding human behaviour.

There are three major problems with all forms of reductionism. First, they involve trying to produce simple explanations within psychology, but by so doing they oversimplify. Second, they are all limited in that they focus on certain factors of

relevance to an understanding of human behaviour while ignoring many other factors. Third, the available evidence doesn’t provide strong support for the reductionist position.

Alternatives to ReductionismThe humanistic approach discussed elsewhere provides one alternative to reductionism. As we have seen, humanistic psychologists such as Maslow and Rogers (see A2 Level Psychology pages 694–695) attached great importance to the self-concept, and to the efforts by humans to realise their potential by means of self-actualisation. Within this approach, there is no systematic attempt to divide the self up into smaller units, or to identify the physiological processes associated with the self-concept.

Most psychologists argue that the humanistic approach to reductionism is too limited. The refusal of humanistic psychologists to consider any kind of reductionism suggests they don’t regard physiological and biological factors as having any real significance. It may be true that each individual’s conscious experience is of importance in understanding his/her behaviour. However, it is likely that other factors also need to be taken into account.

Another alternative to reductionism ishe eclectic approach, in which relevant information is gathered together from various sources and disciplines. Consider, for example, research on the causes of schizophrenia (a serious condition involving hallucinations and loss of contact with reality; A2 Level Psychology Chapter 10, Psychopathology: Schizophrenia). There is evidence that genetic factors are involved. At the biochemical level, some studies have suggested that schizophrenics tend to be unduly sensitive to the neurotransmitter dopamine (see Davison & Neale, 2001). Other evidence indicates that poor social relationships and adverse life events also play a part in producing schizophrenia.

Reductionists might be tempted to produce a biochemical theory of schizophrenia. However, such an approach would involve ignoring environmental factors such as life events. According to the eclectic approach, a full understanding of schizophrenia involves considering all the relevant factors and the ways in which they combine.

The main problem with the eclectic approach is that it is very difficult to combine information from different disciplines into a single theory. For example, how could the concepts of biochemistry be combined with those of life-event research? However, psychology shouldn’t ignore valuable information from other disciplines. This can be seen clearly in recent studies involving brain imaging. Observations of physiological processes in the brain by means of PET and MRI scans are increasing our knowledge of human cognition.

SECTION SUMMARY: ReductionismWhat is reductionism?

Reductionism in psychology is the notion that psychology can ultimately be reduced down to more basic sciences.

There are several forms of reductionism including physiological, experimental, machine, animal, and evolutionary reductionism.

Physiological reductionism Human behaviour can be understood by looking at other more basic

scientific disciplines. Advantages include:

o There is an overlap with psychology and other sciences.o Biochemistry and physiology have well established facts and theories.o Psychological findings should be compatible with physiological

findings. Disadvantages include:

o Human behaviour cannot be understood solely in terms of biological and physiological processes.

o Physiology focuses on structures not processes.o Physiological reductionism does not work well in practice, and

reductionist explanations can contain irrelevant details for psychology.

Experimental reductionism Human behaviour can be understood by looking at simple experiments. Advantages include:

o The experimental method provides the most reliable evidence for human behaviour.

o It is the obvious approach for some theories. Disadvantages include:

o The typical experimental situation may not reflect real life.o The emphasis is on the immediate situation, but other factors can

contribute to behaviour.Machine reductionism

Human behaviour can be understood by comparing ourselves with computers.

Advantages include:o We can reprogram a computer to improve similarities between

computers and humans. Disadvantages include:

o It omits many other factors such as motivation and emotion.o It simplifies the complex human brain.

Animal reductionism Human behaviour can be understood by looking at less complex species. Advantages include:

o Simple forms of learning can be studied more directly in non-human species.

o Observations in other species can have important implications for humans (e.g., learned helplessness).

Disadvantages include:o The human species is very different from other species.o Findings from studies on non-human animals can be misleading in

termsof the human species.Evolutionary reductionism

Human behaviour can be understood by looking at genetics. Advantages include:

o Genetics can be useful to understand individual differences.o It can contribute to an understanding of differences within species.

Disadvantages include:o It is limited in scope.o It doesn’t take into account other factors such as environmental

support.Summary of the advantages and disadvantages of reductionism

Advantages of reductionism as an approach:o All forms of reductionism are useful in that they focus our attention

on factors relevant to an understanding of human behaviour.o They provide assumptions to guide theory and research.

Disadvantages of reductionism as an approach:o All forms of reductionism are oversimplified.o They focus on only certain factors of relevance while ignoring others.

Alternatives to reductionism The humanistic approach:

o It may be true (as humanistic psychologists claim) that conscious experience is important but other factors need to be taken into account.

The eclectic approach:o Psychologists should gather together significant and relevant

information from various sources and disciplines, as exemplified by explanations of schizophrenia, rather than trying to produce a single theory.

NATURE–NURTUREThe so-called “nature–nurture debate” in psychology has a long history, stretching back into philosophical debate about the nature of humankind. The term “nature” refers to behaviour that is determined by inherited factors. “Nurture” is the influence of any environmental factors including learning. The debate is sometimes called heredity versus environment.

Ask yourself: Consider any area in psychology that you have studied. In what way is behaviour in this area of psychology caused by nature or by nurture?

History of the Nature–Nurture DebatePhilosophers have long recognised that aspects of behaviour are inherited. This was long before the discovery of genes by Mendel in the late nineteenth century. Plato, the Greek philosopher, talked about things being inborn or native to an individual, as contrasting with those characteristics that were acquired through experience. This view of inherited characteristics was referred to as nativism.

The opposing philosophical orientation was called empiricism. John Locke in the seventeenth century first outlined the view that all newborn babies are alike. They are born with a mind that is like a blank slate (tabula rasa) and experience records itself in such a way that each individual becomes a unique being. We inherit nothing and all behaviour is acquired as a consequence of experience. The term “empiricism” is derived from “empirical” meaning to discover something through one’s own senses.

Various implications stem from this divergence of opinion. If behaviour is entirely due to heredity then intervention would have little effect on the development of children. In contrast, if all behaviour is learned through experience then the child’s experiences during development are crucial.

Rousseau was an eighteenth-century French philosopher whose work had a major influence on education. He argued that children are noble savages who should be given freedom to follow their innate and positive inclinations. In contrast, the empiricist philosophy suggested that children should be trained in socially acceptable ways. This was the basis for behaviourism and took the view of the child as a passive recipient of instruction.

Nature or NurtureThe use of the term “debate” suggests one must choose between these two opposing views. The nativists and empiricists certainly staunchly supported their different positions but philosophers and psychologists accept that it isn’t an either/or question. There are various arguments that demonstrate this, and it can be best seen in the context of understanding the development of intelligence.

Find out more: Examples of the nature–nurture debate in psychology

Diathesis–stress modelThe diathesis–stress model is based on a similar argument (see your AS studies and the A2 Level Psychology psychopathology chapters). The diathesis–stress model proposes that a complete explanation of any mental disorder is likely to involve both a predisposition to the disorder (an inherited susceptibility to become ill) and a stressor triggering the appearance of the symptoms. This model applies to eating disorders, schizophrenia, and other mental disorders where there is clear evidence of a genetic link from studies of twins, yet not everyone with the gene becomes ill. For example, Holland, Sicotte, and Treasure (1988; see A2 Level Psychology page 211) studied anorexia in identical and non-identical twins. The concordance rate for

identical twins was 56% compared with 5% for non-identical twins (who are genetically less similar). This indicates a high inherited factor, but it is not 100%. We can explain this in terms of the psychological factors that trigger the disorder, such as troubled families or stressful life events. This is an example of nature and nurture interacting.

Ask yourself: In the case of anorexia nervosa, can we say whether nature or nurture is the cause of the disorder?

We can also see the diathesis–stress model at work with the inherited metabolic disorder called phenylketonuria (PKU). In PKU, certain proteins are not processed properly, leaving a poisonous substance in the blood that causes brain damage. If the condition is detected early (and all newborns are tested) then the particular proteins can be eliminated from the child’s diet and there is no brain damage. The question is whether intellectual impairment, should it occur, would be considered as due to nature or nurture. If the child’s environment doesn’t contain the proteins, no damage will occur. Therefore, there is an interaction between nature and nurture.

Genotype and phenotypeThose who believe in the importance of heredity draw a distinction between the genotype and the phenotype. The genotype is an individual’s genetic constitution, as determined by the particular set of genes the individual possesses. Your genotype is your biological or genetic potential to become what you might become. The phenotype is the observable characteristics of an individual, which result from interaction between the genes he/she possesses (i.e., the individual’s genotype) and the environment. Your phenotype is what you actually become as a consequence of the interaction between your biology/genetic make-up and the environment. An example would be hair colour. Your genes determine the colour of your hair, but the fact that you live in a sunny country may mean that your brown hair is bleached in the sun and this produces your blonde phenotype: your observable hair colour, which results from your genetic make-up, and an environmental influence.

As far as intelligence is concerned, we cannot access the genotype. All that can be done is to assess the phenotype by means of administering an intelligence test. This means we never assess inherited abilities except in the context of their environmental expression. There is no such thing as “pure nature”.

The concept of nature presumes that we can isolate an individual who has had no interaction with the environment. People often talk of abilities being present at birth but at this time the human infant has already had 9 months-worth of environmental experience. Even before conception the state of the infant is not all “nature”, as illustrated by something called the transgenerational effect: if a woman has a poor diet during pregnancy her foetus suffers. Perhaps more importantly, if the foetus is female the foetus’s eggs for her own children, which are already formed, will be adversely affected. Therefore the next generation will be underdeveloped because of its grandmother’s poor environment. What may appear to be inherited is in fact

environmentally caused.

This all illustrates the practical difficulties in separating nature from nurture. Hebb (1949) suggested that asking the question of “nature or nurture” is like asking whether a field’s area is determined more by its length or by its width. Of course, its area depends equally on both length and width. In similar fashion, Hebb argued, intelligence depends equally on both heredity and environment. However, while this line of reasoning is valid, we can still reasonably ask whether the areas of different fields vary more because of differences in their lengths or in terms of their widths. In the same way, we can ask whether individual differences in intelligence depend more on differences in genetic endowment or on environmental differences. The question is not nature or nurture, but which one may contribute more. We will consider research methods shortly.

Find out more: Expressing inheritance

The form of interactionIt is often assumed that heredity and environment have independent effects. In fact, however, heredity and environment typically interact in complex ways. For example, consider intelligence. Genetic factors and environmental factors (e.g., the number of years of education) both influence intelligence but not in an entirely separate way—those with the genes for high intelligence also tend to receive the most years of education. As a result, it is very difficult to disentangle the effects of heredity and environment.

Plomin, DeFries, and Loehlin (1977) identified three different kinds of interaction between heredity and environment:

1. Passive heredity–environment interaction. A child’s parents shape the environment in which the child grows up. Intelligent, well-educated parents are likely to have a house full of books and prefer to watch certain programmes on television. This environment is related to the parents’ genetic make-up and thus the parents’ genes are transmitted passively to the child via the environment that the parents create.

2. Reactive heredity–environment interaction. Research has shown that adults do not behave in the same way to a beautiful child as to a “plain” one (Burns & Farina, 1992), and that they find it easier to form a relationship with a child who has an easy temperament than with a child who has a difficult one (Thomas & Chess, 1977). The child’s inherited characteristics (physical attractiveness or temperament) create a reaction in others that leads to differences in the child’s environment. In this way the child’s genetic make-up affects the child’s environment.

3. Active heredity–environment interaction. As each child interacts with his/her environment, the environment is altered and this in turn affects the behaviour of the individual. Bandura called this reciprocal determinismA2 Level Psychology page 231). For example, suppose that Mary has just met

Juliet. If Mary is friendly to Juliet, she may well be friendly in return. This may then lead to a lasting friendship.

Find out more: Where do the main approaches in psychology stand on nature–nurture?

Theoretical Approaches: Piaget and Evolutionary PsychologyAt this point, we will consider in some detail the positions of two different theoretical approaches within psychology to the nature–nurture debate. We will be focusing on Piaget’s theory of cognitive development (which has had huge influence on that area of psychology) and on evolutionary psychology (an approach of increasing importance).

Piaget Piaget’s theory of cognitive development is of interest in connection with the nature–nurture debate. In essence, he accepted that nature and nurture are both important. Piaget assumed that nature played a significant in understanding why all children pass through the same stages of cognitive development in the same sequence. According to his theory, innate, maturational changes within the brain account for the sequence of stages. Piaget claimed (with some justification) that children in every culture show the same general pattern of cognitive development, and he attributed that to innate factors. How does nurture or environment enter the picture in Piaget’s theory? First, the speed with which children proceed through the various stages depends on environmental factors. If the environment (e.g., school) facilitates learning, then children’s cognitive development will benefit. Second, if there is a mismatch between a current experience and the child’s stored knowledge in the form of schemas, this creates a state of disequilibrium or lack of balance. This is an uncomfortable state that motivates the child to acquire new knowledge in order to return to a state of equilibrium. Third, the notion of maturation providing the impetus for children to proceed from one stage of cognitive development to the next implies that most children are definitely in a given stage. Piaget accepted to some extent that children’s specific learning experiences might influence how well they performed certain tasks. For example, the children of Mexican potters seemed to be at a more advanced stage of cognitive development when tasks involved a ball of clay rather than beakers (Price-Williams et al., 1969).

Evolutionary psychologyEvolutionary psychology is an approach that emphasises nature as the main determinant of behaviour. The essence of this approach was expressed clearly by Pinker (1997, p.42):

Natural selection . . . acts by designing the generator of behaviour: the package of information-processing and goal-pursuing mechanisms called the mind. Our minds are designed to generate behaviour that would have been adaptive, on average, in our ancestral environment.

It follows from this approach that key differences between men and women (e.g., in

mate selection; parental investment, in dominance) should be very similar across all cultures. Since changes produced by natural selection typically take thousands of generations to occur, it also follows that key differences between men and women should remain relatively constant within any given culture. The assumptions that many aspects of behaviour are the same across all cultures and that there are only small changes in those aspects of behaviour within cultures both involve de-emphasising the importance of environmental factors in determining behaviour. In other words, these assumptions are incorrect.

Another way in which we can see that the emphasis of evolutionary psychologists is almost entirely on nature at the expense of nurture is in their choice of research topics. For example, consider the following topics: patterns in the maintenance and dissolution of relationships; development of intelligence; social development in adolescence; and bystander behaviour. Nearly everyone accepts that environmental factors are extremely important in all these topics, which may explain why most evolutionary psychologists have steered clear of them!

Researching Nature and NurtureThere is no true experimental evidence in nature–nurture research. Nature–nurture studies compare individuals with the same or different genetic make-up to determine the relative contributions of nature and nurture. Identical twins are genetically the same because they come from a single egg—one zygote. Therefore they are called monozygotic. Non-identical twins come from two zygotes—dizygotic. They are genetically as similar as any siblings, except they share a more similar environment than siblings do right from conception.

Twin studies are a form of natural experiment because the independent variable (genetic relatedness) isn’t directly controlled by the experimenter, and participants aren’t randomly allocated to conditions. It has become clear that, even though identical twins are genetically the same, there are differences from the very moment of conception. This makes it impossible to ever truly investigate the influences of nature versus nurture.

The non-identical nature of identical twinsRecent understanding of genetics has shown us that even cloning will never result in two identical individuals. There are two reasons for this. First of all, due to cell mutation all the cells in a person’s body aren’t identical. Monozygotic twins may start out as identical cells but as these cells divide and multiply to form the living organism, there is some faulty replication, and this leads to minor but possibly significant differences.

Second, small variations in inherited characteristics and in behaviour create different micro-environments. This was the view of Bandura in his concept of reciprocal determinism and is the stance taken by the behavioural geneticist Robert Plomin (1994). In his view each child creates his/her own environment in terms of

how they react to others, how they select interactions, what they attend to and so on.

Twins reared apartIn order to conduct research comparing the effects of nature and nurture in identical twins, studies look at the differences between twins who are reared together or apart. Bouchard and McGue (1981) reported a meta-analysis in which the correlation for identical twins brought up apart was +0.72 compared to +0.86 for identical twins brought up together. The difference in the figures indicates that environmental factors are important, but the fairly high correlation for identical twins brought up apart suggests that genetic factors are also important. However, critics point out that identical twins brought up apart have similar environments and it is true that they are often brought up in separate branches of the same family. That means that it is difficult to interpret the findings.

Shared and non-shared environmentsHarris (1995) raises the question about why siblings and twins, who are raised in the same environments and who have significant genetic similarity, can turn out so differently. Research indicates that about 50% of the variation in most adult characteristics is due to genetic factors. The rest must be environmental, but this cannot be the shared environment because otherwise twins and siblings would be more similar. Furthermore adopted siblings, who share the same environment, would be more similar than they turn out to be—by adulthood there is minimal resemblance between adopted siblings (Maccoby & Martin, 1983).

What is the “non-shared environment”? It cannot be the micro-environment of the child because this too is related to genetic factors, such as physical looks and temperament. The non-shared environment must be influences outside the child’s home. This would explain the fact that twins reared together and twins reared apart show similar correlations in behaviour. The remaining influences are not in the home, i.e., are non-shared, such as peer influences.

Ask yourself: How do your peers influence your development?

Resolving the Nature–Nurture ControversyThe best solution to the nature–nurture controversy may lie in Gottesman’s (1963) concept of a reaction range, similar to the concept of susceptibility in the diathesis–stress model. Our genetic make-up limits the range of our potential development in terms of all characteristics: height, intelligence, mental illness, and so on. Actual development is related to our environmental opportunities, or lack of them. This is the concept of potential (genotype) versus realised potential (phenotype).

Racial differencesWhen we consider the question of whether intelligence is more determined by nature or more determined by nurture, there is one particularly significant issue that arises. This is the question of whether certain groups of people (“races”) are

genetically more intelligent than others. Jensen (1969) produced evidence to demonstrate that, in the United States, black people on average were less intelligent than white people by about 15 points. However, this is an average and about 20% of black people have a higher IQ than that of the average white person.

The difficulty with Jensen’s argument is that between-group differences can be entirely environmental and not genetic at all. Thus, the differences between two different genetic pools (black and white) could be due to environmental and not genetic differences even though within each group genetic factors are important. Consider the following example. If you plant a seed in good soil and provide plenty of sunshine, warmth and food, it thrives. If you plant the identical seed in poor soil with little nourishment it will grow less well. If you plant genetically different seeds in the same soil there will be differences; in this case they are due to nature whereas in the first example the differences are due to nurture. If we compare genetically different groups of people (different racial groups) we must be certain they are sharing the same environment before we attribute the differences to nature, otherwise the differences must be at least in part due to nurture.

Practical and political consequencesThe nature–nurture debate has important practical consequences, as suggested earlier. The interactionist view is that intervention is important. For example, if intelligence is entirely inherited then we should test children as early as possible and place them in suitable schools, and occupations, according to their fixed intellect. If intelligence is influenced by environmental factors, then it is critical that children are given enriching experiences wherever possible to enhance their ability. Whereas psychologists such as Herrnstein and Murray (1994), the authors of The Bell Curve, have argued that individual and group differences in intelligence can never be overcome by interventionist programmes, and therefore it makes economic sense to avoid doing this. Furthermore, they argued that the downward spiral in IQ in the United States could be explained by government subsidies for low-income (low-IQ) mothers.

The political element in the nature–nurture issue shouldn’t be ignored. In the UK, Cyril Burt’s flawed IQ studies were one of the key sources of evidence used to argue in favour of the 11-plus examination and selective placement in secondary schools. (The data were flawed insofar as it later transpired that some of the participants had been invented; see for example Joynson, 1989.) The IQ data suggested that a child’s IQ was a fixed quantity and the educational needs of individuals were better served by having two educational streams: one for brighter pupils and one for less academic pupils. The self-fulfilling prophecy tells us that such division will serve to create inequalities even where none existed previously.

Ask yourself: The concept of the self fulfilling prophecy suggests that expectations can affect development. How is this related to our understanding of nature and nurture?

ConclusionsThe most important conclusion with respect to the nature–nurture debate is that heredity and environment are always both important. Moreover, the two factors interact in complex ways making it difficult to disentangle precisely what each has contributed to an individual’s behaviour. The notion that heredity and environment must both be considered is incorporated into the diathesis–stress model that is very popular in accounting for the factors underlying mental disorders. What is starting to happen is that psychologists are focusing on the kinds of interaction that exist with a view to carrying out relevant research to clarify precisely how these interactions occur. Most theoretical approaches (e.g., Piaget’s theory of cognitive development) accept the notion of interactions between heredity and environment, but a few (e.g., evolutionary psychology) focus almost exclusively on genetic inheritance.

The most direct way of trying to decide on the relative importance of heredity and environment is to study identical and fraternal twins brought up together or apart. Alas, identical twins are not actually totally identical, and identical twins brought up apart tend to experience similar environments. That means that we lack definitive evidence on which to base any conclusions.

The findings from research (especially on intelligence) have sometimes been misrepresented and exploited for political reasons—sometimes by those carrying out the research! This is deplorable and reflects badly on psychology and on those responsible.

SECTION SUMMARY: Nature–NurtureThe history of the nature–nurture debate

Nature:o Refers to inherited characteristics.o Historically, the nativist position was promoted by Plato and

supported by Mendel’s discovery of genetic inheritance. Nurture:

o Refers to the product of experience (environmental influences and learning).

o Empiricism was advocated by philosophers such as Locke, who claimed that at birth we are like a blank slate. We inherit nothing and all behaviour is acquired as a consequence of experience.

The interaction of nature and nurture Nature and nurture interact rather than one or the other determining

behaviour. The cases of phenylketonuria and the diathesis–stress model illustrate this.

The distinction made between genotype and phenotype shows us that we can never actually access the genotype and therefore are always assessing nature and nurture jointly. This can be seen in the transgenerational effect.

There is no such thing as “pure nature”.

The interaction between heredity and environment can be passive, active, and/or reactive.

Theoretical approaches to the nature–nurture debate Piaget’s theory of cognitive development:

o Accepted that both nature and nurture are important. Evolutionary psychology:

o Emphasises nature as the main determinant of behaviour.Researching nature and nurture

Nature–nurture influences are often researched using twin studies, and comparing monozygotic and dizygotic twins.

Problems with this research:o Monozygotic twins are not exactly identical, partly because of small

genetic differences and also because they create their own microenvironments.

o The fact that twins who are reared apart often have similar environments further confounds the data.

The non-shared rather than the shared environment has the greater influence.

Resolving the nature–nurture controversy The concept of a “reaction range” is one way of conceptualising the nature–

nurture interaction. The debate has real-life implications in our understanding of racial

differences in IQ. These are very probably not due to genetic factors because the different groups don’t share the same environment; therefore differences are largely (or entirely) due to environment.

The contribution of nature and nurture has important practical and political implications for interventionist programmes or schemes that separate individuals on the basis of their innate potential.

The most important conclusion is that nature and nurture are both important.

FURTHER READINGThe topics in this chapter are covered in greater depth by A. Bell (2001) Debates in psychology (London: Routledge), written specifically for the AQA A specification. Debates are also discussed in the classic book by E.R. Valentine (1992) Conceptual issues in psychology (2nd Edn.) (London: Routledge). A useful account of the nature–nurture debate is provided by R. Plomin, J.C. DeFries, and G.E. McClearn (1997) Behavioural genetics: A primer (3rd Edn.) (New York: Freeman).

REFERENCES

Bandura, A. (1977). Self-efficacy: Toward a unifying theory of behavioural change. Psychological Review, 84, 191–215.

Bouchard, T.J., & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212, 1055–1059.

Burns, G.L., & Farina, A. (1992). The role of physical attractiveness in adjustment. Genetic, Social, and General Psychology Monographs, 118, 157–194.

Davison, G.C., & Neale, J.M. (2001). Abnormal psychology (8th Edn.). Chichester, UK: Wiley.

Gottesman, I.I. (1963). Heritability of personality: A demonstration. Psychological Monographs, 77 (Whole No. 572).

Harris, W.H. (1995, June). The opportunity for romantic love among hunter-gatherers. Paper presented at the annual convention of the Human Behavior and Evolution Society, Santa Barbara, CA.

Hebb, D.O. (1949). Organisation of behavior. New York: Wiley.

Herrnstein, R.J., & Murray, C.A. (1994). The bell curve: Intelligence and class structure in American life. New York: Free Press.

Hilborn, R.C. (1994). Chaos and nonlinear dynamics. Oxford, UK: Oxford University Press.

Holland, A.J., Sicotte, N., & Treasure, J. (1988). Anorexia nervosa: Evidence for a genetic basis. Journal of Psychosomatic Research, 32, 561–572.

James, W. (1890). Principles of psychology. New York: Holt.

Jensen, A.R. (1969). How much can we boost IQ and scholastic achievement? Harvard Educational Review, 39, 1–123.

Joynson, R.B. (1989) The Burt affair. London: Routledge.

Locke, J. (1690/1989). An essay concerning human understanding. (P.H. Nidditch, Ed.). Oxford, UK: Clarendon.

Maccoby, E.E., & Martin, J.A. (1983). Socialisation in the context of the family: Parent–child interaction. In P.H. Mussein (Ed.), Carmichael’s manual of child psychology: Vol. 4. Socialisation, personality and social development. New York: Wiley.

Motley M.T., Baars, B.J., & Camden, C.T. (1983). Experimental verbal slip studies: A review and an editing model of language encoding. Communication Monographs, 50, 79–101.

Pinker, S. (1997). How the mind works. New York: Norton.

Plomin, R. (1994). Genetics and experience: The interplay between nature and nurture. Thousand Oaks, CA: Sage.

Price-Williams, D., Gordon, W., & Ramirez, M. (1969). Skill and conservation: A study of pottery-making children. Developmental Psychology, 1, 769.

Reber, A.S. (1995). Dictionary of psychology. London: Penguin.

Seligman, M.E.P. (1975). Helplessness: On depression, development and death. San Francisco, CA: W.H. Freeman.

Skinner, B.F. (1971). Beyond freedom and dignity. New York: Knopf.

Thomas, A., & Chess, S. (1977). Temperament and development. New York: Brunner/Mazel.

Valentine, E.R. (1992). Conceptual issues in psychology (2nd Edn.). London: Routledge.

Wachtel, P. (1973). Psychodynamics, behaviour therapy and the implacable experimenter: An inquiry into the consitency of personality. Journal of Abnormal Psychology, 82, 324–334.

Wegner, D.M., & Wheatley, T. (1999). Apparent mental causation: Sources of the experience of will. American Psychologist, 54, 480–492.

Westcott, M.R. (1982). Quantitative and qualitative aspects of experienced freedom. Journal of Mind and Behavior, 3, 99–126.