A Unified Theory of Development: A Dialectic Integration of Nature

and Nurture

Arnold SameroffUniversity of Michigan

The understanding of nature and nurture within developmental science has evolved with alternating ascen-dance of one or the other as primary explanations for individual differences in life course trajectories of suc-cess or failure. A dialectical perspective emphasizing the interconnectedness of individual and context issuggested to interpret the evolution of developmental science in similar terms to those necessary to explainthe development of individual children. A unified theory of development is proposed to integrate personalchange, context, regulation, and representational models of development.

The attention of philosophers and then scientists tohuman development has always begun with a con-cern that children should grow up to be good citi-zens who would contribute to society throughdiligent labor, moral family life, civil obedience,and, more recently, to be happy while making thesecontributions. The motivation for these concernswas that there were many adults who were not.Although attention was paid to the socializationand education of children, it was ultimately in theservice of improving adult performance. The socie-tal concern has always had a life-span perspective.Without healthy, productive adults no culturecould continue to be successful. This concerncontinues to be a major motivator for society tosupport child development research. Although theintellectual interests of contemporary develop-mental researchers range widely in cognitive andsocial–emotional domains, the political justificationfor supporting such studies is that they will lead tothe understanding and ultimate prevention ofbehavioral problems that are costly to society.

With these motivations and supports there havebeen major advances in our understanding of theintellectual, emotional, and social behavior ofchildren, adolescents, and adults. Moreover theseunderstandings have increasingly involved multi-level processes cutting across disciplinary bound-aries in the social and natural sciences. Thisprogress has forced conceptual reorientations asearlier unidirectional views that biological or social

circumstance controlled individual behavior arebecoming multidirectional perspectives where indi-vidual behavior reciprocally changes both biologi-cal and social circumstance.

The models we use to understand how individ-uals change over time have increased in complex-ity from linear to interactive to transactive tomultilevel dynamic systems. Was this progressionin complexity an expression of empirical advancesin our developmental research or is it related tomore general progressions in the history of scienceas a whole? Several years ago during a discussionof a need for a critical social history of develop-mental psychology by a number of distinguishedscientists (Bronfenbrenner, Kessel, Kessen, &White, 1986), Sheldon White argued that it is nec-essary to engage and deconstruct the history of thefield in parallel with efforts to understand thechild. He continued by pointing out that the studyof development needs a self-concept, just as eachchild requires ‘‘the building of some kind of self-referential, self-regulating, self-knowing set ofstructures.’’

If there is a more sophisticated understanding ofthe development of humans, is there a more sophis-ticated understanding of the development of ourscience? The models we use to understand the his-tory of our field from child psychology to develop-mental science should increase in complexity.Understanding developmental science requiresdevelopmental science. And as in the study of any

Correspondence concerning this article should be addressed toArnold Sameroff, Center for Human Growth and Development,University of Michigan, 300 N. Ingalls Building, Ann Arbor, MI48109-0406. Electronic mail may be sent to sameroff@umich.edu.

Child Development, January/February 2010, Volume 81, Number 1, Pages 6–22

� 2010, Copyright the Author(s)

Journal Compilation � 2010, Society for Research in Child Development, Inc.

All rights reserved. 0009-3920/2010/8101-0002

historical process there should be hope that under-standing the past will help us predict the future.

The premise of the general systems theories thatarose in the 1930s was that there were general prin-ciples of organization in every scientific domainthat were at a level of abstraction somewherebetween mathematical formulations and the spe-cific processes being studied (Boulding, 1956). Thishas become apparent in every discipline from phys-ics to political science, as each has moved to modelsof dynamic regulation, where parts cannot be sepa-rated from wholes and useful predictions can onlybe made based on local interactions of multiple sys-tems. The hope of the founders of general systemstheory (cf. von Bertalanffy, 1968) was that scientistswould use a top-down strategy to interpret empiri-cal data from a complexity perspective (Sameroff,1983). This aspiration was not realized because eachscience has tried to be as theoretically simplistic aspossible, resisting the demise of deterministic mod-els until overwhelmed by the complexity of empiri-cal data. The science of psychology has been noexception.

Developmental research aspired to the dicta ofOckham’s razor in the hope of finding simple basicelements and processes that would explain theemergence of life’s complexity. Up through the1960s and into the 1970s statistically significantt tests and analyses of variance gave an illusion thatscience was advancing, but when regression mod-els became dominant and the metric changed tosize of effects (Cohen, 1988), it became clear thatthe field was not doing well at explaining how chil-dren were growing up. Contemporary developmen-talists are quite competent at short-term predictionsof similar cognitive or emotional constructs butmuch worse at the prediction of long-term success-ful life adaptations starting from initial conditions.Increasingly, sophisticated statistical models havebeen sought to separate the behavioral signal ofinterest from the noise of real life. This effort hasled to some frustration in the decreasing amountsof variance that can be attributed to any single fac-tor when everything imaginable is controlled andobscured the possibility that the unexplained vari-ance, the noise, might contain the signals of manyother dimensions of the individual or context thatare necessary for meaningful long-term predictivemodels.

Applicability may not be the most salient criteriafor getting research accepted for publication, but itis highly salient for suggesting ways to changedevelopmental outcomes. The science paid for bythe public is increasingly being asked to meet a

translational rather than a statistical criterion withthe application of research to policy an importantconsideration (Huston, 2008). The primary questionremains as to how we can improve the fate of indi-viduals growing up in our society. To answer thatquestion requires a continuing examination of themodels we need both to study and to understanddevelopment. In what follows I will present a con-temporary summary of what such models shouldcontain and offer a suggestion for an integratedview of development that captures much of the var-iance that needs explaining. No part of what I pro-pose has not been previously suggested by creativeothers. Combining these elements into a unifieddevelopmental theory acknowledges the contempo-rary zeitgeist moving toward more dynamic con-ceptualizations at every level of analysis that istaking place in every other scientific discipline.

A Rough History of the Nature Versus NurtureQuestion

Before complexity was simplicity. For developmen-tal explanations, simplicity was expressed inappeals to aspects of an individual’s nature or nur-ture. The history of developmental psychology hasbeen characterized by swings between opinionsthat determinants of an individual’s behavior couldbe found either in their irreducible fundamentalunits or in their irreducible fundamental experi-ences. The growth process between babyhood andadulthood could be explained either by appeals tointrinsic properties of the child or to extrinsic prop-erties of experience. The nature–nurture questionhas been a central content of developmentalresearch, but it can also be considered to be a majorcontext for developmental research in its appeal todeterministic thinking. As a consequence the his-tory of the nature–nurture question can be used asan organizing construct to understand the historyof our field.

Practically, the nature–nurture question comesinto play when a child has a problem and the ques-tion arises, ‘‘Who is responsible?’’ Most parents’first response is to blame the child and most profes-sionals’ first response is to blame the parents. How-ever, most scientists know that it is both. It is bothchild and parent, but it is also neurons and neigh-borhoods, synapses and schools, proteins andpeers, and genes and governments. But that conclu-sion does not explain how it is both. Do nature andnurture interact deterministically so that the pro-portions attributable to each can be decomposed or

Unified Theory of Development 7

do they transact probabilistically so that the contri-bution of each can only be an abstraction from theactivity of dynamic systems? How this question hasbeen answered in the course of recent history offersa window into how developmental science hasevolved and a perspective on how the question willbe answered in the future.

Since ancient times philosophers have weighedin with their perspectives on the relative influencesof constitution and experience in determining thelife course, but it is in the last few hundred yearsthat these positions have been well articulated,most notably John Locke in the 17th century andRousseau in the 18th. I will begin my rough histori-cal account in the late 19th century with the begin-nings of empirical psychological research in thework of Francis Galton (see Table 1). Francis Galtoncoined the ‘‘nature versus nurture’’ phrase and inhis view inherited characteristics were the originsof human nature. The nurture counterpoint wasmost strongly stated in the work of John Watson inthe 1920s who propounded a new approach helabeled behaviorism, extending Pavlov’s condition-ing processes to explain human individual differ-ences. Learning theory came to dominate humandevelopmental research for almost 50 yearsstrengthened by the operant paradigms promotedin the work of the Skinnerians.

This tilt toward nurture began to shift in the1960s under assault from three directions—ethol-ogy, behavioral genetics, and the cognitive revolu-tion. Where S-R theorists had argued that the lawsof learning were primary in explaining develop-mental change, ethologists were demonstrating thatmany complex behaviors did not seem to need anyreinforcement (Lorenz, 1950) and that S-R contin-

gencies that worked in one species did not work inanother (Breland & Breland, 1961). For example,rats could learn to push a lever to avoid ashock but pigeons could not. Ethologists arguedthat the nature of the species put large restrictionson the effects of nurture such that certainprepared responses were impervious to experience(Seligman, 1970). Statistical advances and data fromlarge samples of twins permitted behavioral gene-ticists to argue that the effects of genes and envi-ronments could be separated, and that very largeproportions of behavioral differences could beexplained by genetic differences (Defries &McLearn, 1973). The cognitive revolution character-ized in the work of Jean Piaget placed the source ofdevelopment in the mind of the child. Experiencewas necessary for the child to construct the worldbut it did not play a role in individual differences.

Where the nativist shift in the 1960s was drivenby advances in biological science, the nurturist shiftin the 1980s was driven by three advances in thesocial science—the war on poverty, the concept of asocial ecology, and cultural deconstruction. Wherebehaviorist research focused on proximal connec-tions between reinforcements and performance, sci-entists in other social disciplines were arguing thateconomic circumstance was a major constraint onthe availability of reinforcements, such that thedevelopmental environments of the poor weredeprived in contrast with those of the affluent. Sim-ilar individuals in different social classes wouldhave quite different developmental outcomes.Bronfenbrenner (1977) in his vision of the socialecology offered a more differentiated model thanprovided by economics alone. He identified thedistal influences of family, school, work, andculture on the availability of reinforcements to thechild, providing a more comprehensive empiricalmodel for predicting individual differences indevelopment. The influence of postmodernistdeconstruction was manifest in the emergence of acultural psychology that went beyond cross-cultural descriptive studies. Meaning rather thanbehavior became dominant through demonstrationsthat the same child behaviors could be givendifferent meanings in different societies leadingto different developmental consequences, andconversely, different behaviors could be given thesame meaning leading to the same consequences.

The new millennium coincided with anotherswing of the pendulum in the nativist direction,again tied to major advances in biological science.Neuroscience and molecular biology have beenmaking major contributions to our understanding

Table 1

Rough History of Nature–Nurture

Historical era Empirical advance

1880–1940s—Nature Inherited differences

Instincts

1920–1950s—Nurture Reinforcement theory

Psychoanalytic theory

1960–1970s—Nature Ethology—species differences

Behavioral genetics

Cognitive revolution

1980–1990s—Nurture Poverty

Social ecology

Cultural deconstruction

2000–2010s—Nature Molecular biology

Neuroscience

8 Sameroff

of development with new technologies for imagingthe brain and manipulating the genome. But, aswill be discussed below, the more recent swingsbetween nature and nurture have been gettingshorter and their intermingling has been increasing.

An examination of Table 1 emphasizes theswings between the popularity of nature and nur-ture as developmental explanations. At each pointin time there are strong adherents of both positionswaiting for some new technological advance toreinforce their point of view. Although this polarityprovides motivation for empirical innovation, it hasthe unfortunate side effect of inhibiting theoreticalinnovation. Despite the alternating claims that theargument is now closed by those on the frontier ofnew explorations of nature or nurture, the factremains that after each advance most of the vari-ance in long-term developmental outcomes is stillunexplained. It is the pressure of unexplained vari-ance that continually negates claims of ascendancyand dialectically motivates continuing exploration.

I have presented a descriptive case for thecycling of explanations between nature and nurtureto raise the question if there is an explanation ofthe repetitive pattern. It could be interpreted assimply the result of technological or theoreticaladvances, but it also could be a phenomenon initself. The development of the nature–nurturedebate might follow developmental principles simi-lar to those that regulate human development andthe examination of the two in parallel might illumi-nate both.

Nonlinear Models of Development

An appreciation of cycling requires an appreciationof a number of nonlinear processes that I will dis-cuss under the general rubric of dialectical theorywith specific attention to a developmental helix andprocesses of differentiation and integration. Dialec-tics have been directly or indirectly emphasized forstudying development and especially relationships(Hinde, 1997; Riegel, 1976). An initial approach todialectics is best captured by consideration of theTaoist diagram of the dark yin and the light yang(see Figure 1) that emphasizes that opposites are ina mutually constituting relationship. They were cre-ated together and remain bound to each other. Thisphilosophical statement is empirically validated atthe most fundamental level of physics wherequarks, the current basic entities, are always in arelationship with each other. At the most funda-mental level of the universe there are no ultimate

units, only ultimate relationships. In the dialecticalyin–yang there is a unity of opposites and an inter-penetration of opposites. The unity is indicated by themutual embrace of the yin and the yang, as seen inthe figure, but yin and yang also interpenetrateeach other as depicted by the small black spot ofyin within the yang and small white spot of yangwithin the yin.

In the psychological realm these ideas have beenapplied frequently, beginning with the philosophi-cal writings of Hegel and most manifest in Piaget’stheory of cognitive development. There is a unityof opposites between one’s cognitions and theworld that is being cognized. Without the worldthere would be nothing to cognize, and without thecognizer there would be no cognitions. But there isalso an interpenetration of opposites. One’s cogni-tion leads to one’s action which becomes part of theworld (the small black dot in the white area), andthen the changed world becomes a part of one’scognition (the small white dot in the black area) ina continuing dialectical progression.

The dialectical perspective on nature and nurtureis that they mutually constitute each other. There isa unity of opposites in that development will notoccur without both, and there is an interpenetrationof opposites in that one’s nature changes one’s nur-ture and conversely one’s nurture changes one’snature, as captured in current transactional models.Moreover, and most salient, without the one, theother would not exist. Species and their environ-ments evolved together in a coactive and transac-tional relationship. Gottlieb’s (1992) construct ofprobablistic epigenesis centered on the joint regula-tion by organismic and experiential factors thatproduced development with neither having priorityover the other. The reciprocal bootstrappingbetween cultural change in groups and cognitive

Figure 1. Unity of opposites and interpenetration of opposites inyin and yang diagram. Nu = nurture; Na = nature.

Unified Theory of Development 9

change in individuals is well articulated by Cole(2006) in his description of human phylogeny.

Although Galton and Watson are the straw menthat nurturists and nativists, respectively, railagainst, both appreciated the unity of constitutionsand environments. Galton (1876) recognized theinfluence of social class and wrote, ‘‘Nature pre-vails enormously over nurture when the differencesof nurture do not exceed what is commonly to befound among persons of the same rank in societyand in the same country.’’ Watson (1914), in turn,recognized that individual and species differenceswere important, ‘‘effectiveness of habit trainingwould be facilitated by knowledge of an animal’sindividual instinctive responses.’’ The unity andinterpenetration of nature and nurture will be morefully explored in the unified model of developmentto follow.

The Developmental Double Helix

The dynamic dialectical interplay between oppo-sites can best be captured as an image of a helixthat depicts the developmental aspects of changesover time as can be seen in Figure 2a. A simpleexample of a developmental progression is thedaily cycle where spiraling to the right would bethe movement toward day and spiraling left wouldbe the movement toward night. Although this is arepetitive cycle, it becomes helical in that each day

is different because of the experience of the previ-ous night and each night is different because of theexperience of the preceding day. A more complexexample would be the development of representa-tion in children (Werner, 1948). Initially, infantsrepresent the world as images of here and nowexperiences. Preschoolers cycle over the same mate-rial but now have the capacity to depict images indrawings that may have a one-to-one correspon-dence to the images but are not the same as theimages. In a few years they will recycle over thesame contents but now with the ability to doabstract representations such as maps where thepictorial aspects may be completely eliminated infavor of words and symbols. Such developmentalrecycling also occurs in the social-emotionaldomain where relationship experiences and repre-sentations derived from early parent–child relation-ships are reworked as children enter into peerrelationships and reworked again in the romanticrelationships beginning in adolescence. Erikson(1959), although not known for his empiricism, wasvery articulate in describing the recycling of iden-tity issues that are never resolved but through abalancing of opposites provide the impetus for eachsucceeding stage. The figure of the helix empha-sizes that the same issues in a variety of domainsare revisited again and again during development.The ubiquity of this helical concept is even foundin Graduate Record Examination practice questions

Figure 2. (a) Developmental helix. (b) Differentiation and integration of helix. (c) Developmental double helix of nature and nurture.

10 Sameroff

(Princeton Review, 2009) where a correct answer is‘‘Science advances in a widening spiral in that eachnew conceptual scheme embraces the phenomenaexplained by its predecessors and adds to thoseexplanations.’’

Differentiation and Hierarchic Integration

The developmental helix pushes us toward amore elaborate nonlinear process expressed as dif-ferentiation and hierarchic integration. As formu-lated in Werner’s (1957) orthogenetic principle,‘‘Wherever development occurs it proceeds from astate of relative globality and lack of differentiation toa state of increasing differentiation, articulation, andhierarchic integration.’’ If viewed within the helicalmetaphor in Figure 2b, we could consider themovement toward differentiation as going in onedirection with a widening of the coil, as for exam-ple, the number of words in a child’s vocabulary orthe number of color concepts increases, and thenthe movement toward integration going in theother direction with a narrowing of the coil, as forexample, the chunking of metacognition occurs,only once again to begin differentiating again as thenumber of metaconcepts increases.

If we consider the historical differentiation ofnature, what began in Galton’s laboratory as a cata-logue of measurable differences in behavior wasreconceptualized as really being differences in neu-rological electrical activity, and then as really beingdifferences in neurotransmitter activity, and then asreally being differences in genomic activity, andmost recently as really being differences in epige-nomic activity.

Analogously, there was also a historical differen-tiation of nurture where an early romantic con-ception of the power of mother love wasreconceptualized as differences in the pattern ofreinforcements provided by the parent, and thenreconceptualized when it was discovered thatdifferences in social circumstance constrained thepatterns of reinforcement available to the child, andthen reconceptualized when social circumstancewas differentiated into the subsystems of the child’ssocial ecology, and then reconceptualized when itwas realized through social deconstruction that theeffects of social ecology were constrained by themeanings that families and cultures imposed onbehavior.

The progression of nature and nurture concep-tions can be summarized by a double helix thatcaptures their alternating differentiation and inte-gration waxing and waning through time (see

Figure 2c). Each new breakthrough initially goesthrough a stage of differentiation as a new method-ology comes into play and then integration as itbecomes connected to developmental phenomena.The developments in molecular biology would be arecent example on the nature side where the gen-ome project produced the differentiated genes thatnow can be integrated into endophenotypes thathave more proximal connections to behavior. Onthe nurture side, the differentiation of the socialecology into a set of subsystems of family, school,peer group, and neighborhood influences, forexample, led to efforts at integrating its effect ondevelopment within comprehensive statistical mod-els. Whether one gains ascendance over the other isa complex result of psychology (e.g., it is easier toconceptualize the parts we are made of than thewholes of which we are parts), anthropology (e.g.,the preference in Western culture for individual-based rather than relationship-based explanationsof behavior), sociology (e.g., whether there is agreater societal demand to mitigate the effects ofbiological disease or social disorder), and econom-ics (e.g., whether investments in nature or nurtureresearch offer the best opportunity to reduce thecosts of developmental problems).

What is important in this discussion is to appre-ciate that there is a cycling between nature andnurture explanations of development that have adevelopmental course. The development of our sci-ence may be very similar to, and thus very usefulfor, understanding the development of humanbeings. The dialectics of differentiation and hierar-chic integration may characterize all developmentalprocesses.

We can come away from this discussion withone of two propositions. The first is that the cyclingbetween nature and nurture will continue untileither one or the other gets it right effectively end-ing the argument. Unfortunately, the problem ofmultifinality and equifinality undercuts this possi-bility (Cicchetti & Rogosch, 1996). On the natureside, whatever measure of individual differenceshas been discovered, two children with the samecharacteristics can have quite different outcomesand two children with different characteristics canhave the same outcome. On the nurture side, what-ever measure of the social environmental has beendiscovered, two children with the same experiencescan have different outcomes and two childrenwith quite different experiences can have the sameoutcome.

The second proposition is that nature and nur-ture represent a unity of opposites such that neither

Unified Theory of Development 11

can ever get it right on its own. Because of theirinterpenetration advances in our understanding ofnature illuminate nurture and changes in ourunderstanding of nature illuminate nature.Although the literature contains many references tothe fact that one cannot separate nature from nur-ture, there are fewer references to how their unityoperates. The rest of this article will be devoted toan effort to integrate contemporary advances in ourunderstandings of both nature and nurture into aunified theory of development.

A Unified Theory of Development

Contemporary developmental science requires atleast four models for understanding humangrowth: a personal change one, a contextual one, aregulation one, and a representational one. The per-sonal change model is necessary for understandingthe progression of competencies from infancy on. Itrequires unpacking the changing complexity of theindividual as he or she moves from the sensorimo-tor functioning of infancy to increasingly intricatelevels of cognition; from early attachments with afew caregivers to relationships with many peers,teachers, and others in the world beyond home andschool; and from the early differentiation of selfand other to the multifaceted personal and culturalidentities of adolescence and adulthood. The contex-tual model is necessary to delineate the multiplesources of experience that augment or constrainindividual development. The growing child isincreasingly involved with a variety of social set-tings and institutions that have direct or indirectimpact as exemplified in Bronfenbrenner’s (1977)view of the social ecology. The regulation modeladds a dynamic systems perspective to the relationbetween person and context. During early develop-ment, human regulation moves from the primarilybiological to the psychological and social. Whatbegins as the regulation of temperature, hunger,and arousal soon turns to regulation of attention,behavior, and social interactions. The last is the rep-resentational model where an individual’s here andnow experiences in the world is given a timelessexistence in thought. These representations are thecognitive structures where experience is encoded atabstracted levels that provide an interpretive struc-ture for new experiences, as well as a sense of selfand other. Combining these four models offers acomprehensive view of the multiple parts, wholes,and their connecting processes that comprisehuman development.

Personal Change Model

Because psychology’s central focus is on individ-uals, developmental psychology’s main concernshave been on how children change over time. Howone thinks about developmental change will have aclear influence on research objectives. Three waysof conceptualizing change can be seen inFigure 3—trait, growth, and developmental. If onebelieves that an individual consists of a set ofunchanging traits then there is no need for develop-mental research. If development is considered agrowth process then it can have classic epigeneticexplanations in that all the parts are there to startwith and it is their interactions that produce thechanges in the phenotype, or it can be consideredexperience dependent but only as nutrition for theunfolding maturation process. Viewing personalchange as a stage process can have a descriptive ortheoretical meaning (Kessen, 1962). Descriptivestages are paraphrases for age and consist of lists ofaverage achievements, for example, of 1-year-olds,

Figure 3. Personal change depicted as a trait, growth, ordevelopment.

12 Sameroff

2-year-olds, or 3-year-olds, similar to how intelli-gence quotient (IQ) tests are constructed. In con-trast, the theoretical use of stage implies that thereis a period of stability of functioning followed by atransition to a structurally different period of stabil-ity presumed to reflect more encompassing cogni-tive and social functioning. The classic examples oftheoretical use of stages are in the writings of Freudand Piaget. Although there have been major revi-sions or rejections of these particular formulations,there are some generally accepted notions thatwithin many domains individuals move fromnovices, to experts, to masters where they do notjust do things better, they do things differently(Ericsson & Charness, 1994).

The general range of developmental changes hasbeen extended well into adulthood and aging bythe orientations of life span (Baltes, 1979) and lifecourse theories (Elder, 1979) with their heavyemphasis on the importance of continuing altera-tions in the family, the workplace, and the histori-cal epoch as individuals move into adulthood. Theinability to separate individuals from context in thelife-span models of adulthood provides a motiva-tion to reconceptualize the importance of develop-mental context for younger individuals as well. Thechild or individual is not a unity and any modelof the person also has to include the complex ofpsychological and underlying biological changesas well.

Contextual Model

Although developmental psychology is focusedon individuals, it has become clear that under-standing change requires an analysis of an individ-ual’s experience. Behavior, in general, anddevelopment, in particular, cannot be separated

from the social context. Our understanding of expe-rience has moved from a focus on primary caregiv-ers to multiple other sources of socialization. Therewere many predecessors who felt that families,schools, neighborhoods, and culture had influenceson development, but Bronfenbrenner turned theseideas into a comprehensive framework with predic-tions of how these settings affect the child but alsohow they affect each other. Although his terminol-ogy of microsystems, mesosystems, macrosystems,exosystems, and chronosystems may not be univer-sally accepted, his principles that the family, school,and community are all intertwined in explainingany particular child’s progress is now universallyacknowledged (see Figure 4).

Traditionally, social contacts were considered toexpand from participation wholly in the family mi-crosystem into later contact with the peer groupand school system. Today, however, many infantsare placed in out-of-home group child care in thefirst months of life. Each of these settings has itsown system properties such that their contributionsto the development of the child are only one ofmany institutional functions. For example, theadministration of a school setting needs attention tofinancing, hiring, training of staff, and buildingmaintenance before it can perform its putative func-tion of caring for or educating children (Maxwell,2009). Thus, a sociological analysis of such settingsprovides information about its ability to impactchildren.

Attention to the effects on children of changingsettings over time must be augmented by attentionto changing characteristics of individuals within asetting. Contemporary social models take a lifecourse perspective that includes the interlinked lifetrajectories of not only the child but other familymembers (Elder, Johnson, & Crosnoe, 2003). Forexample, experience for the child may be quite dif-ferent if the mother is in her teens with limitededucation, or in her 30s after completing profes-sional training and entry into the job force.

Capturing the complex effects of multiple envi-ronmental situations has been a daunting enterpriserequiring vast sample sizes to capture the uniquecontributions of each setting. An alternative meth-odology to dimensionalize the negative or positivequality of a child’s experience has been the use ofmultiple or cumulative risk or promotive factorscores. For example, a set of data on the effects of anumber of environmental variables on adolescentdevelopment was provided by a study of a largegroup of Philadelphia families (Furstenberg, Cook,Eccles, Elder, & Sameroff, 1999).Figure 4. Social-ecological model of context.

Unified Theory of Development 13

In the Philadelphia project 20 environmental fac-tors were assessed and combined to approximatean ecological model containing six contextual sub-systems. These were Family Processes that includedsupport for autonomy, behavior control, parentalinvolvement, and family climate; Parent Characteris-tics that included mental health, sense of efficacy,resourcefulness, and level of education; FamilyStructure that included the parents’ marital statusand socioeconomic indicators of household crowd-ing and welfare status; Family Management com-posed of variables of institutional involvement,informal networks, social resources, and adjust-ments to economic pressure; Peers that includedindicators of association with prosocial and antiso-cial peers; and Community that included censustract information on average income and educa-tional level of the neighborhood, a parent report ofneighborhood problems, and measures of the ado-lescent’s school climate. In addition to the largenumber of ecological variables, we used a widearray of youth developmental outcomes in fivedomains: Psychological Adjustment, Self-Competence,Conduct Problems, Extracurricular Involvement, andAcademic Performance.

For the environmental risk effects analyses eachof the 20 variables was dichotomized with approxi-mately one fourth of the families in the high-riskgroup and then the number of high risk conditionssummed. When we examined the relation betweenthe multiple risk factor score and the five adoles-cent outcomes, there were large declines in out-come with increasing risk and a substantial overlapin slope for each (Sameroff, 2006). Although thiskind of epidemiological research does not unpackthe processes by which each individual is impactedby contextual experience, it does document themultiple factors in the environment that are candi-dates for more specific analyses.

We also examined the effects of promotive influ-ences in the Philadelphia study. Sameroff (1999)proposed that a better term for the positive end ofthe risk dimension would be promotive rather thanprotective factors. A promotive factor would have apositive effect in both high- and low-risk popula-tions, which is far more common than a protectivefactor that only facilitates the development of high-risk children. We created a set of promotive factorsby cutting each of our environmental variables atthe top quartile, rather than the bottom, and sum-ming them. The effects of the multiple promotivefactor score mirrored the effects of the multiple riskscore. Children from families with many promotivefactors did substantially better than children from

families with few promotive factors on each of ourarray of adolescent outcomes. For the youth in thePhiladelphia sample, the more risk factors, theworse the outcomes, and the more promotive fac-tors, the better the outcomes. In sum, contextincludes a constellation of environmental influencesthat have general effects on child development, fos-tering child development at one end and inhibitingit at the other.

Of great significance for the life course, theseeffects play out over time as a manifestation of theMatthew effect, ‘‘To the man who has, more will begiven until he grows rich; the man who has not willlose what little he has’’ (Matthew 13:12). In a studyof high- and low-IQ 4-year-olds we tracked theiracademic achievement through high school(Gutman, Sameroff, & Cole, 2003). The low-IQgroup living in low contextual risk conditionsconsistently did better than the high-IQ groupliving in high risk conditions. Over time promotiveor risky contextual effects either fostered or wipedout prior individual competence.

Regulation Model

The third component of the unified theory is theregulation model reflecting the systems orientation ofmodern science (Sameroff, 1983). The idea that thatthe child is in a dynamic rather than passive rela-tionship with experience has become a basic tenetof contemporary developmental psychology. How-ever, most of the rhetoric is about ‘‘self’’-regulation.Whether it is Piaget’s assimilation-accommodationmodel in cognition or Rothbart’s (1981) reactivityand self-regulatory view of temperament, equilibra-tion is primarily a characteristic native to the child.The context is necessary as a source of passiveexperiences that stimulate individual adaptation,but has no active role in shaping that adaptation.These views promote a belief that regulation is aproperty of the person. However, self-regulationmainly occurs in a social surround that is activelyengaged in ‘‘other’’-regulation. At the biologicallevel the self-regulatory activity of genes is inti-mately connected to the other-regulatory activity ofthe surrounding cell cytoplasm. In Thelen’s (1989)view of dynamic systems other-regulation is pro-vided by the strange attractors of chaos theory. Theself-regulation leading to an infant’s neurologicallybased coordination of walking is constrained by theother-regulation of the child’s muscle development,the strange attractor.

This issue of the developmental expansion ofself-regulation to include other-regulation is

14 Sameroff

captured by the ice-cream-cone-in-a-can model ofdevelopment (Sameroff & Fiese, 2000) depicted inFigure 5. The developmental changes in therelation between individual and context are repre-sented as an expanding cone within a cylinder. Thebalance between other-regulation and self-regula-tion shifts as the child is able to take on more andmore responsibility for his or her own well-being.The infant, who at birth could not survive withoutthe caregiving environment, eventually reachesadulthood and can become part of the other-regula-tion of a new infant, beginning the next generation.

It is parents who keep children warm, feed them,and cuddle them when they cry; peers who providechildren with knowledge about the range andlimits of their social behavior; and teachers whosocialize children into group behavior as well asregulate cognition into socially constructeddomains of knowledge. Although these other-regu-lators can be considered background to the emer-gence of inherent individual differences inregulatory capacities, there has been much evidencefrom longitudinal research among humans andcross-fostering studies in other animals that ‘‘self’’-regulatory capacities are heavily influenced by theexperience of regulation provided by caregivers.The capacity for self-regulation arises through theactions of others. This regulation by others providesthe increasingly complex social, emotional, andcognitive experiences to which the child must self-regulate and the safety net when self-regulationfails. Children’s cognition to a large extent is notderived from direct experiences with the environ-ment but based on interpretations provided by oth-ers (Gelman, 2009). Moreover, these regulations areembedded not only in the relation between childand context but also in the additional relations

between family and their cultural and economic sit-uations (Raver, 2004). These regulatory systemsrange from the here-and-now experiences of par-ent–child interactions to governmental concernwith the burden of national debt that will be passedon the next generation and to conservationists’ con-cerns with the fate of the planet as a viable environ-ment for future generations of humans.

Early functional physiological self-regulation ofsleep, crying, and attention are augmented by care-giving that provides children with regulatory expe-riences to help them quiet down on the one handand become more attentive on the other. Sleep is aninteresting example where biological regulationbecomes psychological regulation through socialregulation. As wakefulness begins to emerge as adistinct state it is expanded and contracted by inter-actions with caregivers who stimulate alertness andfacilitate sleepiness. Although it remains an essen-tial biological process, eventually it takes on a largedegree of self-regulation as the child and then adultmake active decisions about waking time and sleep-ing time. But this agentic decision making remainsintimately connected with other-regulation in termsof the demands of school and work for specificperiods of wakefulness.

Robert Emde and I with a group of colleagues(Sameroff & Emde, 1989) in an attempt to describemental health diagnoses for infants argued for aposition that infant diagnoses could not be sepa-rated from relationship diagnoses. Our point wasthat in early development life is a ‘‘we-ness’’ ratherthan an ‘‘I-ness.’’ The developmental and clinicalquestion in this case is when does diagnosisbecome individualized, at what stage does a childhave a self-regulation problem instead of an other-regulation problem? One answer is to identify thepoint in development when areas of self-regulationbecome independent of initial regulatory contextsand are carried into new relationships. Childrenwho have imaginary playmates provide an interest-ing perspective on the relation between self- andother-regulation. The more preschoolers engaged infantasy and pretense, the more sophisticated theirtheory of mind (Taylor & Carlson, 2009).

Generally, research into self-regulation hasfocused on part processes, such as emotion orattention. Such empirical isolation obscures thelarger picture in which many interacting systemsare playing significant roles. Without regulationprovided by the social context, for example,nutrition and temperature, the young child wouldnot survive to engage in emotional or attentionalprocesses.

Figure 5. Transactional relations between self-regulation andother-regulation.

Unified Theory of Development 15

Transactional Regulation

The previous discussion of the need for a con-struct of other-regulation to complete an under-standing of self-regulation leads now to how therelation between self and other operates develop-mentally and for this we turn to the transactionalmodel (Sameroff & Chandler, 1975). Transactionsare omnipresent. Everything in the universe isaffecting something else or is being affected bysomething else. In the transactional model thedevelopment of the child is a product of the contin-uous dynamic interactions of the child and theexperience provided by his or her social settings.What is core to the transactional model is the ana-lytic emphasis placed on the interdependent effectsof the child and environment and is depicted in thebidirectional arrows between self and other inFigure 5.

In a recent book on the topic (Sameroff, 2009), anumber of researchers documented transactionalprocesses in cognitive and social-emotionaldomains where agents in the family, school, andcultural contexts altered the course of children’sdevelopment in both positive and negative direc-tions. Transactional examples have been typicallyin the behavioral domain with an emphasis on par-ent–child mutual exacerbations producing problembehavior in both partners (Patterson, 1986). Morerecently, transactions have been recognized in tea-cher–student relationships where the effects of theteacher on the child in one grade will change thereaction of the teacher in the next moving the stu-dent to higher or lower levels of competence(Morrison & Connor, 2009). Multilevel transactionshave also been documented where not only theparent and child are transacting with each otherbut both are also transacting with cultural practices(Bornstein, 2009).

Vygotsky’s (1978) zone of proximal developmentis analogous to transactional other-regulation incognitive development. Successful socialization andparticularly good education is based on fittingexperience to the developmental status of the child.As children create their understanding of theworld, the world is made more complex throughsteps in a curriculum to move them along towardsome societal goal of mature thought. Arithmetic isan excellent example where as soon as childrenlearn to add, they are required to learn to subtract,following which they are taught to multiply anddivide. Each step is a transactional regulation of theenvironment by the teacher to keep one step aheadof the child’s mathematical regulation. Similarly, in

the social realm increases in social responsibilityare paced to the success of the adjustment to previ-ous levels of responsibility (Rogoff, 2003).

In a more popular vein Gladwell (2008) describesthe life course of a number of eminent individualsin sports, commerce, and technology, whereequally competent children did not achieve similargreatness because of the lack of social, educational,or technological possibilities. In each case initialadvantage scaffolded the child to be able to elicitand make use of a series of opportunities docu-menting the transactional progression that eventu-ally led to eminence.

Representational Model

Representations are encodings of experience.They are a more or less elaborated internal sum-mary of the external world. They include the cogni-tive representations where the external world isinternalized, the social representations where rela-tionships become working models, the culturalrepresentations of different ethnicities or social clas-ses, and the developmental theories discussed here.Representations are obviously not the same as whatthey represent. They have an adaptive function ofbringing order to a variable world, producing a setof expectations of how things should fit together.

We have long been familiar with such represen-tations as perceptual constancy in which objects areperceived as being a certain size even when thesensory size is manipulated. In such a summationcertain aspects are selected and others ignored. Inthe representation of a square for example, the size,color, and texture of the square object may beignored. Analogously, when representations aremade of a social object such as a parent, certainfeatures are included in the representation andothers are ignored. Research using the adult attach-ment interview (Main & Goldwyn, 1984) has foundthat representations of parents are often idealized,where only positive aspects are included in themental model. Although the links between thequality of representations of child–parent relation-ships during infancy and those during adulthoodare far from direct, early working models of attach-ment do seem to have long-term consequences foradult development (Sroufe, Egeland, Carlson, &Collins, 2005).

Similarly, parents create representations oftheir children that emphasize certain aspects,deemphasize others, and have stability over timeindependent of the child’s actual characteristics.We had parents rate their infants’ temperament

16 Sameroff

during the 1st year of life following a structuredinteraction sequence (Seifer, Sameroff, Barrett, &Krafchuk, 1994). We also had them rate the temper-ament of six unfamiliar infants engaged in the sameinteraction sequence. The average correlation intemperament ratings of the unfamiliar infantsbetween mothers and trained observers was .84with none below .60. The average correlation intemperament ratings between mothers and trainedobservers for their own children was .35 with arange down to ).40. Mothers were very good ratersof other people’s children but very poor raters oftheir own due to the personal representations thatthey imposed on their observations. Documentingsuch differences in parent representations would beof no more than intellectual interest, if there werenot consequences for the later development ofthe child. For example, infants whose mothersperceived them as problematic criers duringinfancy increased their crying during toddlerhoodand had higher problem behavior scores when theywere preschoolers (McKenzie & McDonough, 2009).

Individual well-being is also a result of meaning-ful cultural engagement with desirable everydayroutines that have a script, goals, and values(Weisner, 2002). Meaningfulness, a key componentof cultural analyses, is primarily found in coherentrepresentations. Evidence of a positive effect ofmeaning systems can be found in Fiese andWinter’s (2009) descriptions of how family routinesprovide a narrative representation for the rest ofthe family members that allows the whole to con-tinue adaptive functioning despite the variability inthe behavior of the parts. Evidence of a negativeeffect of lack of meaningfulness is in a study ofnative Canadian youth who showed much higherlevels of suicide and other problem behavior whenthere were large inconsistencies in cultural continu-ity from one generation to another (Chandler, Lal-onde, Sokol, & Hallett, 2003). The order or disorderin a family or society’s representation of itselfaffects the adaptive functioning of its members.

Unifying the Theory of Development

Now that the four models necessary for a theory ofdevelopment have been described, I can proceed tointegrate them into a comprehensive view that con-tains most known influences on life trajectories.I will begin with a structural depiction of thecomponents of the personal and contextual modelscontaining all the pieces relevant to development.I will then add the regulation and change compo-

nent of the personal model to capture the processesthat produce the life course and then finish the uni-fied theory with an overlay of the representationalmodel.

Structural Formulation

The self is composed of a set of interacting psy-chological and biological processes. The psychologi-cal domains overlap in cognitive and emotionalrealms of intelligence, mental health, social compe-tence, and identity, among others. These aredepicted as the set of grey, overlapping circles com-prising the psychological part of the self inFigure 6. Each of these psychological domains issubserved by and interacts with a set of interactingbiological processes, including neurophysiology,neuroendocrinology, proteomics, epigenomics, andgenomics that are depicted as a set of black, over-lapping circles. Together the gray and black circlescomprise the biopsychological self system. Thisself-regulation system interacts with the other-regulation system, depicted by the surroundingwhite circles, representing the many interactingsettings of the social ecology, including family,school, neighborhood, community, and overarchinggeopolitical influences. Taken together thethree sets of overlapping circles comprise the bio-psychosocial aspects of the individual in context.

Process Formulation

The process formulation adds the personalchange time dimension to the biopsychosocialmodel, which can be viewed as either a growthmodel, where the biopsychological aspects increasequantitatively over time but there is no change intheir interrelationships as in the cone image (seeFigure 5) or a developmental model, where theaspects have qualitative shifts in organization inwhich there are changing relations among the bio-psychosocial aspects (see Figure 7).

Evolutionary theory has provided a fruitful ana-log for understanding the transitions that lead fromone developmental stage to another. As opposed tothe gradualist understanding of evolutionarychanges originally proposed by Darwin that wouldlook like the growth model, Eldredge and Gould(1972) argued that evolution was characterized bycontinuity evidenced in long periods of stasiswhere there were only modest changes alternatingwith discontinuity where there were short periodsof rapid change that they labeled punctuated equilib-rium. The implication was that there was a balance

Unified Theory of Development 17

between species and their ecosystems until it wasinterrupted by either large changes in the speciesor large changes in the environment that required anew equilibration. In terms of understanding devel-opmental discontinuities in the individual, wewould need to search for such changes in the childor the context that create pressures for a new equili-bration. These forces are represented by the upand down arrows around points of inflection inFigure 7.

One of the most commonly accepted transitionshas been the 5- to 7-year shift in cognition origi-nally documented in 21 behavioral domains byWhite (1965) and accentuated in the work of Piaget.Thirty years later Sameroff and Haith (1996) and agroup of contributors reexamined this transition

but also asked if there were contextual changesduring this age period. We reached the conclusionthat there was a 5- to 7-year shift in the child if by 5we meant 3 and by 7 we meant 10. This answerreflects the study of what might be called ‘‘partprocesses.’’ If one asks whether 5-year-olds canattend, remember, have emotions, engage in socialinteractions, and even take charge of social interac-tions, the answer is yes. If one asks whether 5-year-olds can fully integrate their physical, cognitive,emotional, and social worlds, the answer is no. Butneither can 7-year-olds. So what is the punctuationbetween the ages of 5 and 7? On average 5- to7-year-olds can integrate several behaviors thatpermit the beginnings of formal education in mostcultures in the world—increased cognitive ability,the ability to sit still, and the ability to pay atten-tion. Some children have these capacities muchearlier, but the requirements for successful partici-pation in the school setting require all three plus anumber of others. White’s (1996) more recent con-clusion was that, ‘‘what happens to childrenbetween 5 and 7 is not the acquisition of an abso-lute ability to reason; it is an ability to reason withothers and to look reasonable in the context of soci-ety’s demands on the growing child to be coopera-tive and responsible (p. 27).’’ In Figure 7 there areup arrows from self to other reflecting childadvances, but there may be more powerful influ-ences from other to self where society does thedevelopmental punctuation by requiring the childto spend most of the day in school rather than athome. From this perspective the stages of infancy,childhood, adolescence, and adulthood could be

Figure 6. Biopsychosocial ecological system.

Figure 7. Unified theory of development including the personalchange, context, and regulation models.

18 Sameroff

relabeled the home stage, the elementary schoolstage, the secondary school stage, and the workand new family stage.

Similar analyses can be applied to the punctua-tions that occur in the transition to adolescence oradulthood. It is the relation between shifts in thechild and shifts in the context that marknew stages. Puberty is a biological achievement ofthe child but adolescence is a socially designatedphase between childhood and adulthood(Worthman, 1993). Puberty is universal but adoles-cence is not, either in historical or cross-culturalperspective. In many cultures adolescence isdirectly tied to biological changes but in modern-izing cultures it is more closely tied to age-basedtransitions into middle and high schools. Depend-ing on the culture sexual participation can beencouraged at an early age before biological matu-rity or discouraged until individuals are well intoadulthood. These pressures from changes in thechild and the context are represented by the upand down arrows around the adolescent transitionin Figure 7. In western societies, adolescenceis generally recognized but the quality of the ado-lescent experience is quite variable and may beheavily dependent on stage–environment fit.Depending on the particular family or school sys-tem, desires for autonomy and intimacy can befostered or thwarted moving the adolescent intobetter or worse future functioning. Negative psy-chological changes associated with adolescentdevelopment often result from a mismatchbetween the needs of developing adolescents andthe opportunities afforded them by their socialenvironments (Eccles et al., 1993).

The unified theory depicted in Figure 7 combinesthe personal change, contextual, and regulationmodel, but it would become overly complex to addthe representational model to the figure, as well.Suffice it to say that representation suffuses everyaspect of the model in the interacting identities,attitudes, beliefs, and attributions of the child, thefamily, the culture, and the organizational structureof social institutions. Moreover, the way develop-mental science conceptualizes the child may beonly one of a number of possible cultural inven-tions (Kessen, 1979). The most important represen-tation for current purposes is captured in thedepiction of a unified theory of development.Like most theories the unified view does not makespecific predictions but does specify what will benecessary for explaining any developmentalphenomena. It is a reversal of the usual bottom-upempirical stance where the researcher maintains as

narrow focus as possible unless forced to enlargethe scope by some contradictory findings. The top-down theoretical stance is that researchers need tobe aware that they are examining only a part of alarger whole consisting of multiple interactingdynamic systems.

Future of Nature Versus Nurture

Current Nature Ascendance

The current ascendance of research using newbiological measures of individual differences is theresult of the interdisciplinary collaboration thatParke (2004) had indicated was essential to theadvance of developmental research. These advancesin molecular genetics, endocrinology, and neurologyare being rapidly integrated into psychologicalresearch. The good news is that the new science isno longer based on the reductionist models of thepast where linear progressions were proposedbetween biological entities such as genes or neuro-transmitters and psychological function. In eachdomain multidirectional models are replacing unidi-rectional ones with a growing emphasis on gene–environment interactions, epigenome–experiencetransactions, and brain plasticity. These advancesare relationship based, requiring increasingly com-plicated systems analyses to capture the multiplepart–whole processes underlying developmentalchange. Nurture, for example, the environment ofthe gene, the environment of the cell, and the envi-ronment of the organism, are incorporated intoadvanced analyses of the contribution of context atevery level of analysis. It is striking that the nonre-ductionist systems thinking that those who definepsychology as a natural science have avoided is anow a central part of their colleague disciplines ofbiology and physics. Developmental science is bene-fiting from advances in the natural sciences at thetheoretical as well as the empirical level.

Next Resurgence of Nurture

A renewed emphasis on the importance of nur-ture is underway. Again, it is a dialectical result ofthe inability of appeals to human nature to explainfully developmental pathways. There remain largeamounts of unexplained variance. The nurtureresurgence is implicit in the new directions for bio-logical sciences such as epigenomics, describedabove, and will become explicit with a more power-ful appreciation of the perspectives on humandevelopment provided by social sciences beyond

Unified Theory of Development 19

psychology. The core element in each interdisciplin-ary effort is that successful developmental predic-tions from psychological measures are highlycontingent on the social or biological context.Two of the major ingredients needing integrationinto a unified developmental science are theopportunity structure construct from sociology andeconomics and the meaning making construct fromanthropology.

The important perspective that sociology adds todevelopmental science is that individuals areembedded in networks of relationships that con-strain or encourage different aspects of individualbehavior. Social institutions like families, schools,and the workplace are composed of roles that chil-dren come to understand and fill. In this view indi-vidual differences, the core of psychologicalconcern, are limited by role demands in predictingdevelopmental outcomes. Economists are interestedin what keeps economies going and individualbehavior is viewed through the lens of financialchoices. The part of economics most relevant tobehavioral development is the availability of anopportunity structure. Once again the predictivepower of individual differences is constrained bythe availability of such resources as educationalsystems, job choices, and social mobility that deter-mines whether individuals have the option to usetheir prior competencies or not. Anthropology isindeed interested in cultural differences in behav-ior, but equally important for understanding devel-opment are differences in meaning systems, that is,how different cultures think about their practices.The same behavior can have quite different mean-ings and quite different behaviors can have thesame meaning in different cultures. Again the pre-dictive power of individual differences is con-strained by how different cultures value andproscribe different behaviors.

Development of the Developmentalist

I began this article proposing that the study ofthe development of our field would illuminate ourstudy of the development of individuals. Up untilthe 1960s child psychologist was the predominantlabel for researchers with children and the mainfocus was on identifying measures of stable intelli-gence and personality traits that would be predic-tive of adult performance. In the 1960s and 1970swe became developmental psychologists as organiza-tional principles and emergents dominated therhetoric around the cognitive revolution and attach-ment theory. During the 1980s and 1990s we

reframed ourselves as developmental scientists whenwe gained a fuller appreciation of the contributionof biology and the social ecology to psychologicalgrowth. In the new millennium we again arechanging our self description to developmental sys-tems theorists as multilevel biopsychosocial dynamicsystems are becoming the framework for under-standing human change over time and statisticiansare providing tools that are closer approximationsto the complexity of our data.

With regard to what we have learned aboutnature and nurture, the future challenge is not tofind new arguments for one or the other but tocreate a developmental model where advances inthe study of both individual and context areexpected and hoped for. I have proposed such abiopsychosocial unified theory of developmentthat I hope will be useful for future research inhuman development. Over time the body changes,the brain changes, the mind changes, and theenvironment changes along courses that may besomewhat independent of each other and some-what a consequence of experience with each other.It should be a very exciting enterprise to fill inthe details of how biological, psychological, andsocial experiences foster and transform each otherto explain both adaptive and maladaptive func-tioning across the life course.

Coming full circle to the dialectical principlesof the yin–yang model, there are continuities asscientists concerned with greater differentiationswithin our biological and social experience con-tinue to push our understanding of both natureand nurture. But there are increasing discontinu-ities with the rhetoric of the past as many moredevelopmentalists realize that neither nature nornurture will provide ultimate truths and neithercan be an end in itself. Instead, each can explainthe influences of the other because in the end nei-ther can exist without the other. They mutuallyconstitute each other through their unity andinterpenetration of opposites. The schematic depic-tion of the unified theory of development providesan integrated way of looking at things, but alsofor things. Although we all have a strong desirefor straightforward explanations of life, develop-ment is complicated and models for explaining itneed to be complicated enough to usefully informour understanding.

Everything should be as simple as possible, butnot simpler.

Albert Einstein

20 Sameroff

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