facial expressions 1 running head: humor, tickle, and pain

Facial Expressions 1

Running head: HUMOR, TICKLE, AND PAIN

Facial Expressions, Smile Types, and Self-report during Humor, Tickle, and Pain:

An Examination of Socrates’ Hypothesis

Christine R. Harris

Psychology Department

University of California, San Diego

Nancy Alvarado

Center for Human Information Processing

University of California, San Diego

Author Contact: Christine R. Harris, Ph.D. Department of Psychology - 0109 University of California, San Diego La Jolla, CA 92093-0109 Phone: (858) 822-4507

Email: [email protected]

Word Count (main text and footnotes): 4028


Key words: emotion, facial expressions, laughter, humor, pain, tickling


Abstract

The nature of ticklish smiling and the possible emotional state that accompanies it have been

pondered since the ancient Greeks. Socrates proposed that tickle induced pleasure and pain. Others,

including Darwin, have claimed that ticklish laughter is virtually the same as humorous laughter. The

present study is the first to systematically examine facial behavior and self-reports of emotion in

response to tickling. Using a within-subjects design, 84 subjects’ responses to being tickled were

compared to their responses when experiencing a painful stimulus and their responses to comedy.

Overall results for both self-report and facial action coding showed that the tickle condition elicited both

pleasure and displeasure. Facial action during tickling included “Duchenne” smiles plus movements

associated with negative emotions. Results suggest that tickle-induced smiling can be dissociated from

positive affect. Tickle may be a type of complex reflex or fixed-action pattern.


Facial Expressions, Smile Types, and Self-report during Humor, Tickle, and Pain:

An Examination of Socrates’ Hypothesis

Tickling and the smiling it induces, at first blush, seem like child’s play. However, tickle, along

with crying and startle responses, falls into the category of poorly understood affective phenomena.

One intriguing aspect of tickle is that a physical stimulus can elicit a response that appears to closely

resemble that of humor. The apparent similarity of the smiling and laughter induced by tickle and by

humor has led many writers to assume that the two reflect the same internal state. One champion of this

view was Charles Darwin (1872), who noted that “the imagination is sometimes said to be tickled by a

ludicrous idea; and this so-called tickling of the mind is curiously analogous with that of the body”

(p.199). Darwin described several similarities between tickle and humor. First, he claimed that a

pleasant hedonic state was necessary to elicit laughter: “in this case, [humor] and in that of laughter from

being tickled, the mind must be in a pleasurable condition; a young child, if tickled by a strange man,

would scream in fear” (p.199). Second, he noted similarities in the elicitor of both states, “The touch

must be light” in tickle and “an idea or event must not be of grave import” in humor. Finally, he pointed

out that an element of surprise is required to elicit laughter to jokes or to tickle. In essence, tickle was

simply a physical joke. Much contemporary writing on the topic echoes the view that ticklish laughter

and smiling is the result of a positive affective state (e.g. Keith-Spiegel, 1972; Panksepp, 2000;

Weisfeld, 1993).

In contrast, Francis Bacon (1677) proposed that when tickled “men even in a grieved state of

mind, yet cannot sometimes forbear laughing” (p. 151). This view suggests that ticklish smiling and

laughter can be dissociated from a humorous state. Moreover, Bacon suggested that “tickling is ever


painful, and not well endured” (p.161). The possibility that tickle elicits an unpleasant state was noted as

far back as the ancient Greeks. Socrates suggested that tickling elicited pleasure but to a greater degree

pain (cited by Gregory, 1924).

Despite centuries of speculation, only a handful of empirical studies have investigated tickle,

most within the past few years. For example, neuroscientists have employed light tickling in fMRI

studies to explore the neural activation of anticipation of sensory stimuli (Carlsson, Petrovic, Skare,

Petersson, & Ingvar, 2000; Blakemore, Wolpert, & Frith 1998). The remaining researchers have

focused on trying to understand the nature of tickle.1 Observations such as those by Bacon have led

some to propose tickle-induced smiling may have little to do with a positive emotional state such as

amusement or happiness. Instead the tickle response may be a type of complex reflex or fixed action

pattern (e.g. Black, 1982; Harris, 1999; Stearns, 1972), or perhaps a species-typical stereotyped

motor pattern that requires a particular releasing stimulus (Provine, 1997). Recent findings provide

some support for this view. First, while previous work has shown a warm-up effect of humorous stimuli

(the first few jokes in a series are not as funny as later jokes), warm-up does not seem to transfer

between humorous stimuli and tickle (Harris & Christenfeld, 1997). Second, ticklish laughter and

smiling does not require the belief that another person is performing the tickling. Responses were as

intense when people were alone and believed they were being tickled by a machine as when they were

being tickled by another person (Harris & Christenfeld, 1999). Third, in Pavlovian conditioning, tickling

has served as the unconditioned stimulus that elicits laughter (Newman, O’Grady, Ryan, & Hemmes,

1993) and tickle-induced smiling emerges in infants even when it has been not paired with playful

interactions (Leuba, 1941).


Despite this burgeoning interest in tickle, several questions remain unanswered. What is the

connection between the facial expression of tickle and the internal subjective state? Is the smiling

elicited during tickling completely disconnected from the internal emotional state as suggested by Bacon

and advocates of a reflex explanation? Is the facial expression a mixture of pleasure and pain as

claimed by Socrates? Do people who find tickle pleasurable display different facial expressions than

those who find it unpleasant?

Present Research

The aim of this study was to explore these basic questions. The speculation that the tickle

response is due to amusement has been based primarily on the assumption that the smiling that occurs

during tickling is indeed the same as that occurs during humor. However, while only four of the studies

described above specifically measured smiling/laughter during tickling; none of them closely examined

these nonverbal displays to determine whether they differed from those evoked by humor.

This study offers the first microanalysis of the facial movements that occur during tickling. It also

systematically examined the emotional state that people reported experiencing during tickling, an aspect

neglected in previous work. A within-subjects design was used to examine behavior and self-reported

emotional responses of 84 subjects across three conditions: (1) being tickled for 10 seconds, (2)

listening to a series of recorded comedy routines, and (3) immersing one hand in a bucket of circulating

cold water for as long as tolerable. Comparing the facial expressions and self-report of the experience

of tickle is of interest in its own right, and may shed light on the previously mentioned hypotheses

regarding the nature of tickle. Including painful and humorous conditions enables us to examine whether

tickle simply resembles amusement or has painful or unpleasant aspects.


Method

Subjects

Eighty-four healthy University of California, San Diego students (61 females), who had not

taken pain medicine, participated to fulfill a class requirement.

Procedures

Each subject participated alone and was exposed to three conditions: tickle, comedy, and pain.

Order was counter-balanced across subjects. After each condition, the subject filled out an emotion

inventory. Facial behavior was recorded by a videocamera placed in front of the subjects. Only the

lens of the camera was exposed; the rest of the camera was hidden behind a large board, which

contained colorful pictures.

Tickle Condition. The female research assistant stood behind the subjects and tickled them on

their sides (from the armpits to the waist) for 10 seconds. Subjects were informed that they could stop

the tickling if the sensation became too intense. No subject asked to stop.

Comedy Condition. Subjects listened to an audiotape of a series of twelve jokes by various

comedians (e.g. Seinfeld, Cosby). The first 2 jokes were used as practice stimuli.

Pain Condition. The subject’s left hand was immersed in a bucket of circulating cold water (1-5

Cº). Subjects were instructed to press a button when they first started to feel pain and then to press it

again when they could no longer tolerate the pain, at which point they withdrew their hand from the

water. If the subject had not pressed the intolerable pain button after 3 minutes, the experimenter

terminated the task. One subject continued for the entire 3 min.


Dependent Measures

Facial Action Coding. Subjects’ facial movements were coded using Ekman and Friesen’s

Facial Action Coding System (FACS; 1978). FACS provides a method for objectively describing

movement in the face. It catalogs distinct combinations of observable muscle movements into 44

numbered appearance changes called “action units” (AUs). Ten seconds of facial behavior were coded

during each condition as follows: the 10 sec during tickling, the 10 sec immediately before removing the

hand from the cold water; and one second following each of the punchlines of the ten jokes (a total of

10 sec).

Self-report ratings. After experiencing each condition, subjects described their subjective states

using a series of rating scales labeled with the names of emotions and feelings. They used a scale of 0

(not at all) to 7 (extremely) to rate how much they had felt: anxious, happy, angry, embarrassed, and

amused. They similarly rated how much they found the experience unpleasant and painful. In the tickle

condition, subjects also rated how ticklish and unpleasant they found the sensation of being tickled. At

the end of the experiment subjects were asked, “Do you enjoy being tickled?”

Results and Discussion

Results of analysis of self-report ratings and facial activity are described separately, then

compared. There were no sex differences on any of the measures except that men had higher

amusement ratings and lower pain ratings. Women rated the tickle as more ticklish and unpleasant.

Analysis of Self-report Ratings

Self-reported ratings of the emotional state elicited during tickling (See Figure 1) appear to

generally support Socrates’ suggestion that tickle is capable of eliciting both positive and negative affect.

Paired two-tailed t-tests of each rated emotion were used to compare ratings during tickle to those


during pain and humor. Happy and amused ratings for the tickle condition were significantly higher than

those reported in the pain condition (t (83) = 9.70, p < .001; t (83) = 5.57, p < .001, respectively) and

significantly lower than those reported in the humor condition (t (83) = 3.87, p < .001; t (83) = 5.11, p

< .001, respectively). Anxious and unpleasant ratings for the tickle condition also fell between the pain

and humor conditions; tickle elicited significantly higher ratings than humor stimuli (t (83) = 5.67, p <

.001; t (83) = 4.03, p < .001, respectively) and significantly lower ratings than the pain stimulus (t (83)

= 3.49, p < .002; t (83) = 19.52, p < .001, respectively). However, subjects described tickle more

often as unpleasant rather than as painful. The tickle condition elicited virtually no self-reported anger

but did elicit significantly more embarrassment than the pain (t (83) = 8.07, p < .001) or humor (t (83) =

8.29, p < .001) conditions.

In short, based on people’s self-reports of their emotional experience, tickle does not appear to

simply elicit the emotional state of amusement. Relative to the humor and pain conditions, tickle elicited

an intermediate level of both positive and negative affect across individuals.

Analysis of Facial Behavior

Two analyses of facial behavior were performed. First, occurrence of individual AUs was

analyzed. Second, smiles were classified by smile type and these types were compared across the three

conditions.

Individual AUs. Is facial behavior consistent with the suggestion of Socrates and Bacon that

tickle elicits pain as well as pleasure? While withdrawal movements have been noted in support of this

claim, no one has previously examined the face to determine whether it reflects the experience of

unpleasantness. Table 1 displays the mean occurrence of each type of AU per event. An event is

defined as a set of action units overlapping in time forming a configuration held on the face together


before fading. Ekman (1997) suggests that the event, not the individual AU, is the appropriate unit of

analysis. For example, co-occurrence of AU1 and AU4 typically characterizes distress expressions

(e.g., fear, sadness). Inner brow raises (AU1) together with a lowered brow movement (AU4),

accounted for a significantly greater number of the AUs in the pain condition than in the other two

conditions.

In support of Bacon and Socrates, several AUs were more common during both pain and tickle

compared to humor. These include wrinkling the nose (AU9) and raising the upper lip (AU10), both of

which are associated with the expression of disgust (Rozin, Lowery & Ebert, 1994). The lip stretch

(AU20), which gives the appearance of a grimace, also was a more typical movement during pain and

tickle, as was the lip press (AU24) and tightening of the eyelids (AU7). The present findings, disclosing

that tickle and pain share some facial movements, provides behavioral support for the view that tickling

elicits some negative affect or discomfort. Several of the AUs that were found to be proportionally

more frequent in the pain and tickle conditions in the current work have also been found in previous

work on pain. Prkachin (1992) examined facial behavior in response to four different types of pain

elicitors. He concluded that four actions reflect a general pain expression: levator contraction

(AU9/10), orbit tightening (AU6/7), brow lowering (AU4) and to a lesser extent eye closure (AU43).

Tickle shared movements not only with pain but also with humor. Lips parting/ jaw dropping

(AU 25/26) was more frequent in humor and tickle than in pain. AUs that comprise the Duchenne smile

(AU6 and AU12) accounted for more of the movements in the humor and tickle conditions than in the

pain condition. Past research sometimes combines the activation of AU6 and 7 (since they both

function to tighten the region around the eye). However, the distinction between the two was kept in the

current work since activation of 6 is important in the Duchenne smile but is also found in other emotions


such as distress and sadness while AU7 can occur with anger and with fear (Ekman & Friesen, 1982).

In the present work, AU6 was more frequent during tickle and humor while AU7 was more

characteristic of pain and tickle.

Smile Types. To determine whether tickle-induced smiling qualitatively differed from humor-

induced smiling, we examined the degree to which different AUs co-occurred with smiling across

conditions. Types of smiles were divided into four categories based upon the AUs observed. All smile

types included AU12 and opening of the mouth and jaw (AU25/26). The first category was labeled a

non-Duchenne smile, and consisted of pulling the lip corners back (AU12) without any other AU

activity. The second category was the Duchenne smile, defined by the actions of AU6 + AU12 (cheek

raise added). This has been considered the smile of felt pleasure or the “enjoyment smile” (Ekman &

Friesen, 1982; Ekman, Davidson, & Friesen, 1990). The third category was a controlled smiled,

defined as an AU12 together with another lower facial movement whose action functioned to counteract

or obscure the smile. This was defined primarily as turning the lips down (AU15) or pressing the lips

together (AU24). The fourth category was a mixed smile, defined as an AU12 occurring in the

presence of AUs that are commonly associated with negative affective states (e.g. AU10, AU1+4, or

AU20). As shown in Figure 2, the pattern of smile types varies across conditions. Duchenne smiles

were found both in the tickle and humor conditions, but never in the pain condition (χ²= 56.84, df = 2,

p < .001). Mixed smiles were the most frequent type of smile in the pain condition (51%) and were

relatively common in the tickle condition (24%), but were relatively infrequent in the humor condition

(10%): χ²= 81.35, df = 2, p < .001. The proportion of non-Duchenne and controlled smiles did not

significantly differ across the three conditions.


Given that Duchenne smiles and mixed smiles were the two types of smiles that showed a

different pattern across conditions, the next analyses focused on the relationship of these two smiles with

self-report of emotion. In the tickle condition, Duchenne smiles were correlated with amused ratings (r

= .24, p < .03) but were more highly correlated with finding the tickle sensation ticklish (r = .29, p <

.008) and unpleasant (r = .31, p < .005). Amused and unpleasant ratings were not correlated, (r = -

.07), suggesting that during tickling Duchenne smiles were associated with positive internal states for

some subjects and negative internal states for others.2 Mixed smiles, like Duchenne smiles, were

correlated with finding the tickle sensation ticklish (r = .27, p < .02) and unpleasant (r = .32, p < .004)

and were also correlated with happy ratings (r = .25, p < .02). Happy ratings and unpleasant ratings

were not correlated (r = -.05).3

In the humor condition, Duchenne smiles correlated with happy (r = .25, p < .03) and amused

ratings (r = .32, p < .005), replicating previous work by Ekman and colleagues. Mixed smiles were

also correlated with ratings of happy (r = .36, p < .002) and amused (r = .22, p = .05). In the pain

condition, there were no Duchenne smiles and mixed smiles were not significantly correlated with any of

the emotion rating scales.

Interpretation of Results

The findings that Duchenne smiles never occurred in the pain condition and were significantly

correlated with amusement and happiness self-reports in the humor condition are consistent with

previous work that has argued that the Duchenne smile is the smile associated with positive affect.

Smiles during the pain condition were relatively infrequent; when these did occur, they were most likely

to take the form of a mixed smile. One possible interpretation of mixed smiles is that they may be what

Ekman and Friesen (1982) have referred to as masking smiles in work examining negative emotions.


They suggested that "In a masking smile strong negative emotion is felt and an attempt is made to

conceal those feelings by appearing positive" (p.244). In support of this interpretation, our study found

that mixed smiles in the pain condition were not correlated with any of the self-report measures of

emotion. Mixed smiles in the context of unambiguous pain may be the result of an individual’s coping

style rather than a direct readout of an emotional state (Alvarado & Harris, 2001).4

The present findings showed that tickling is clearly capable of eliciting the Duchenne smile,

which is normally associated with a positive hedonic state. However, mixed smiles were as common in

the tickle condition as were Duchenne smiles. One possibility is that tickle elicits positive affect that then

gives rise to smiling. In the case of the mixed smiles, the positive affect would be superimposed over the

negative state elicited by the discomfort of tickling, suggesting a blend or mixture of emotions.

However, our findings weigh against this interpretation: Duchenne smiles and mixed smiles were both

correlated with finding the tickle sensation unpleasant and with positive affect ratings. Yet,

unpleasantness ratings were not correlated with positive affect. In other words, some people showed

Duchenne smiles and mixed smiles even though they did not report feeling happy or amused.

An alternative view -- based on the hypothesis that tickle is a type of complex reflex (Harris,

1999) -- is that the smiling associated with tickling is not the result of a positive hedonic state. Instead,

it is an automatic response that is elicited by a physical stimulus much like the kick elicited by tapping

below the knee. According to this view, the negative AUs found during mixed smiles reflect the

discomfort elicited during tickling just as they do during pain.5 However, the smile (AU12), whether

imbedded in a Duchenne smile or a mixed smile, is an automatic behavior that is not reflective of any

emotional reaction. This interpretation is supported by an additional finding of dissociation between

smiling and self-reported pleasure. Open-ended responses to “Do you enjoy being tickled?” were


categorized as yes (32%), mixed/neutral (32%), or no (36%). The three groups did not differ in

number of Duchenne and mixed smiles. If anything people who dislike tickle produced slightly more

Duchenne smiles (M = 1.21) than those who report liking it (M = .89).

Is Tickle a Reflex or Fixed Action Pattern?

The current findings support the view that tickle is a type of complex reflex or fixed action

pattern. Interestingly, tickle shares several features with another phylogenetically old reflex, startle. One

can no more tickle oneself than startle oneself. Both seem to require some element of surprise. Another

similarity is that startle and tickle both appear to elicit facial expressions that resemble expressions that

are usually elicited during emotional states. However, just as Ekman, Friesen, and Simons (1985) have

proposed that the startle expression is not the result of an emotional state, we suggest that ticklish

smiling does not arise from any positive emotional state.

One might wonder why such a reflex or fixed action pattern would exist, a question that seems

more straightforward for startle, with its orienting and defensive functions. One hypothesis is that

tickling elicits smiling because it serves as a ready means for fostering a bond between caregiver and

infant (Alexander, 1986; Fridlund & Loftis, 1990). The child smiles in response to tickling which causes

the parent to smile which in turn elicits further smiling from the child. This reciprocity of smiling leads

both parent and child to experience positive affect. This view, however, does not explain why the

sensation of tickle also elicits a negative physical sensation.

The unpleasant aspect of tickle has led some writers to suggest that the most ticklish areas are

those that are most vulnerable in hand to hand combat, thus tickling confers an adaptive advantage by

motivating one to protect these areas (Gregory, 1924; Hall & Allin, 1897). This would explain the

pulling away and fending off movements frequently encountered during tickling. These researchers did


not discuss why tickling elicits a facial expression that is usually associated with positive affect. One

possibility is that the tickle smile may function as an appeasement display.

Harris (1999) drawing on different aspects of these views recently proposed that tickle may

elicit smiling to encourage conspecifics to perform the tickling and may elicit negative affect in the one

being tickled in order to motivate the developing primate to avoid the tickling. By thus encouraging

rough and tumble play, it may promote development and acquisition of combat skills. This view is

consistent with Provine’s (2000) interesting suggestion that laughter first emerged from rough and tumble

play in primates and only later became associated with humor in humans. Although these various views

offer some intriguing suggestions, testing hypotheses regarding ultimate mechanisms is notoriously

difficult. However, what does seem to be emerging from current research is that tickle is not merely a

special case of amusement.

In closing, we note that the present conclusion that ticklish smiling is not the result of a positive

affective state does not mean that no positive emotions ever arise during tickling. Various aspects of the

social setting (such as who does the tickling, how long it lasts, the pre-existing mood of the person

tickled) are capable of eliciting a variety of reactions, some enjoyable and some unpleasant. For

example, the common practice of “tickle torture”, in which several children hold down and tickle one

child, can elicit anger whereas two new lovers tickling one another can promote enjoyment (see

Provine, 2000 for descriptive data). Some of the confusion that has arisen over the relationship of

smiling and laughter and affect during tickling may reflect an erroneous assumption that these positive

aspects of the situation are responsible for the smiling observed in tickling. The present results, along

with recent findings mentioned in the introduction, lend support to the notion that ticklish smiling may


have no closer a connection to mirth and merriment than crying when cutting onions has to sorrow and

sadness.


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Footnotes

1There may be two types of tickle and the connection between the two types is unknown (Hall & Allin,

1897; Harris, 1999). Knismesis feels like a “moving itch” and is elicited by a light, moving stimulus.

The present work focuses on gargalesis, the laughter-inducing tickle elicited by a rapid, higher pressure

stimulus applied to certain ticklish areas.

2 Schimmack (2001) noted that the absence of a correlation may not be a strong test of whether two

emotional states can co-occur. He suggested an alternative test that examines the minimum intensity of

the two states. If the states are incompatible, the value should approach 0. The min value of the

present data is .67, suggesting that in this circumstance individuals did not report experiencing high levels

of both states during the same period.

3 Min value = .61.

4 Mixed smiles were infrequent in the humor condition (accounting for less than 10% of the smiles).

When such smiles did occur, they were correlated with positive affect. One possibility is that mixed

smiles in pain and in humor have a different source. In humor such smiles may arise from positive affect

accompanied by negative evaluation of the comic stimuli (e.g. a joke that was funny but inappropriate).

5 In the tickle and pain conditions, the correlation of pain and unpleasantness ratings with negative AUs

did not present a clear picture. Due to space limitations, these data are not presented here. Other

researchers have noted a lack of correlation between self-report and specific pain AUs and have

suggested that different measures may assess different aspects of pain (Craig et al., 1991; Patrick,

Craig, and Prkachin, 1986).


Table 1

Mean Occurrence of AUs Per Event Across Conditions

Condition*

Action

Pain Tickle Humor

AU1: Inner Brow Raise

.11ª .06 .06

AU4: Brow Lower

.09ª .05 .04

AU6: Cheek Raise

.01 .24ª .28ª

AU7: Lids Tight

.08ª .08ª .02

AU9: Nose Wrinkle

.03ª .05ª .01

AU10: Upper Lip Raise

.10ª .08ª .05

AU12: Lip Corner Pull

.14 .61ª .57ª

AU15: Lip Corner Depress

.07 .16ª .10

AU20: Lip Stretch

.06ª .05ª .01

AU24: Lip Press

.09ª .11ª .06

AU25/26: Lips Part, Jaw Drop

.20 .38ª .37ª

AU43: Eyes Closed

.07ªº .10ª .05º

*Values with a shared superscript (a or o) are not significantly different from each other. Only values without a common superscript differ significantly (p < .05), two-tailed t-test.


Figure Captions

Figure 1. Self-report Ratings of Emotion.

Figure 2. Percent of Smile Types (relative to all smiles) in Each Condition

Figure 3. Examples of Facial Displays During Each Condition


Emotion Term

Painful

Unpleasant

Angry

Anxious

Embarrassed

Amused

Happy

Mea

n

6

5

4

3

2

1

0

PAIN

TICKLE

HUMOR


Type of Smile

MixedControlledNon-DuchenneDuchenne

Per

cent

60

50

40

30

20

10

0

PAIN

TICKLE

HUMOR

facial expressions 1 running head: humor, tickle, and pain

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