Laughter down-regulates negative emotional arousal amongst friends [version 1; peer review: 1 approved]

Laughter is contagious, sensitive to social context, and can be used to mitigate negative emotional states. This experiment tested whether moments of transition from negative to positive affect were associated with more laughter if in familiar compared to unfamiliar company. 90 participants (47 females, mean age 20.61 years), either familiar (N=42) or unfamiliar (N=48) to the principle researcher, were randomly assigned to listen to 44 seconds of music which induced a fearful affect, positive affect or a neutral mood, followed by 30 seconds of infectious laughter (N=30/group). Filmed facial expressions were coded for four dependent variables (duration: half smile; full smile; laugh , and extent: peak mirth ) of amusement in response to the laughter. Familiar participants fully smiled for longer than unfamiliar participants ( F (1,84)=4.15, p =.045). There was an affect-familiarity interaction for peak mirth ( F (2,84)=4.68, p =.01), time spent half smiling F (2,84)=5.00, p =.009), and fully smiling ( F (2,84)=3.48, p =.035). Post hoc analyses revealed familiar participants exhibited greater peak mirth and smiled (half and full) for longer than unfamiliar participants in the fearful affect condition. Laughter and positive emotions may be used to moderate negative arousal more amongst people known to each other than amongst strangers. The paper is interesting, and innovative. It also has scientific support to enrich the clinical psychologists in science and in practice. The procedures are very well presented and analyzed in the results and discussion. theoretical good classical literature. are well presented. However, the English needs review.


Introduction
Laughter is an inherently social behaviour which is prevalent in interactions as wide ranging as tickling and conversations (Gervais and Wilson 2005;Provine 2001). Laughter is not automatic, rather highly modulated by social factors. Humorous material, for example, is rated more amusing when participants believe others have also found it amusing (Wimer & Beins, 2008).
Within interactions, laughter regulates mood and coordinates the behaviour of laughter through a group (Spoor and Kelly 2004;Deacon 1997;Provine 1992). Affect and laughter are induced in a listener both through the directly arousing acoustic properties of laughter, and indirectly as a conditioned positive affective social experiences (Owren and Bachorowski 2003). Laughter shows an in-group function: the act of laughing increases positive affect which further enhances the perceived pleasure of social interactions, promoting affiliative and cooperative behaviour (Owren and Bachorowski 2003;Martin 2007;Smoski and Bachorowski 2003). Participants laughed and smiled more, laughed longer, and rated humorous material more favourably when they heard in-group laughter as opposed to out-group laughter or no laughter at all (Platow et al. 2005).
The "undo" hypothesis theorises that positive emotions, including laughter and smiling, function to mitigate negative arousal that arises from stressful events (Levenson 1988). Cardiovascular arousal, including increased heart rate and blood pressure associated with a fearful film, dissipated more quickly when followed by happy films as compared to sad or neutral films (Fredrickson and Levenson 1998). Music-induced positive emotions preceded by the viewing of unpleasant pictures restored baseline cardiovascular and respiratory levels faster than did white noise (Sokhadze 2007).
The aim of the present study was to combine these two separate explorations of laughter and address the use of laughter to regulate negative emotional affective states amongst people known to each other. The study investigated whether more laughter and positive emotional expressions were produced by participants in familiar than unfamiliar company, in the context of transition from negative arousal to a more positive affect. Music was used to induce an affective state (positive or negative) or a non-affective state using non-arousing music as a control group. The music presentations were followed by presentation of infectious laughter. The hypothesis was that laughter produced in response to infectious laughter would be greatest following fearful affect, and would show sensitivity to social context, being greatest in familiar compared to unfamiliar company.

Design
Amusement in response to the infectious laughter was the dependent variable. Manipulated mood (positive affect, negative affect, control) was the independent variable in a between-subjects design. Participants were randomly assigned, using a standardized table of random numbers on a 1:1:1 ratio, to one of the three experimental groups: 1) fearful affect; 2) positive affect; 3) neutral, ensuring 30 participants in each group. 17 participants were known to the experimenter in the fearful affect group, 11 in the positive affect group and 14 in the neutral group (Table 1). The assigned condition was written in pencil on the back of the printed consent forms.

Acoustic stimuli Music induction excerpt
A single suitable musical excerpt for each target mood appropriate for the target population was chosen from a large pool of potential stimuli using a pilot study (N = 6). The final fearful and positive affect excerpts were rated as those capable with inducing the strongest fearful and positive affect, with equal arousal ratings on an 11-point arousal scale (0-not arousing 10-highly arousing). The chosen stimuli were: Half Remembered Dream, by Hans Zimmer (Fearful affect), Monsier Verdoux Paris Boulevard, by Charlie Chaplin (positive affect). The control song Sleepy Noise, by Canton (neutral), was chosen based on its rating as non-arousing in the pilot study. Track tempo has been linked to arousal (Etzel et al. 2006) therefore the two emotionally arousing tracks were equated in tempi using Logic Studio's Time Machine and AUpitch algorithms which leave the pitch intact. The fearful track reached a natural crescendo in 44 sec and pilot testing confirmed this was adequate to elicit the target emotional response, therefore equivalent sections in positive affect and control track were located and cut using Logic studio. The laughter tracks were included unexpectedly at the end of the audio tracks.

Laughter tracks
A pilot study was used to select the most contagious laughter sequences from a selection collaborated from the internet (www.freesound.org) and from live recordings from a social gathering recorded using a Dictaphone. Laughs from people of different sex and age were used to ensure ecological validity. The final selection contained 19 laughs which had been successful in eliciting laughter in all pilot participants.
This laughter track was edited onto the end of each musical excerpt, after a one second gap, using Logic Studio, to create three separate audio tracks for each experimental condition, each lasting 1 minute 15 seconds. All audio experimental stimuli were presented binaurally through headphones, and played off an iPod classic at a volume of 3.4 decibels.

Procedure
Participants were tested individually. The testing location varied, but consistency was maintained through ensuring participants were always seated at a desk, in a quiet room with only the principle experimenter present. Participants were informed the experiment was an 'Investigation into effects of acoustic stimuli on facial musculature' and instructions, presented in written form, asked participants to listen to a short acoustic extract whilst their facial reactions were recorded on camera. The facial muscle movements of interest, they were told, were out of conscious and therefore they should not suppress or force any movement, just concentrate on the sounds they would hear. Participants consented to being filmed and blindfolded. Music induction procedures commonly ask participants to close their eyes to aid acoustical concentration (Etzel et al. 2006). Participants were instead blindfolded to alleviate participants of this responsibility. Once participants were blindfolded and wearing headphones, positioned ½ m from the Camera Recorder, the experimenter opened a numbered word document which revealed the assigned experimental condition and played the audio track appropriate to this condition. Facial expressions were continuously recorded. At the end of the audio, recording was suspended, headphones and blindfold were removed, and participants completed a questionnaire which asked them to "select one word that best represents the emotion elicited in you by the musical part of the acoustical excerpt" and whether they recognised the musical excerpts. Participants were then debriefed, and fully informed consent obtained.

Coding
The videotaped recordings were digitalised and the 30 sec of behavioural responses from the onset of the laughter track were isolated for analysis. Playback was performed on iTunes Movies. Following previous research, amusement was measured using coding sheets along two dimensions: intensity and duration, which have shown maximum sensitivity to subtle differences in mirthful expressions (Platow et al. 2005). All facial image analysis coding was performed by independent observers (six for intensity of mirth, two for humour durations), who were blind to the experimental condition of participants and to the experimental hypotheses. Each observer was familiarised with two practice trials before analysis commenced.

Peak Mirth
A Mirth scale was created based on scales used in previous research. The Mirth Index created by Zigler and colleagues (Zigler, Levine, and Gould 1966)  All six categories were found easily discernible in a pilot study. Six observers were asked to mark for each participant the highest intensity level of mirth observed within the 30 seconds of participant recordings.

Mirthful durations
The total time each participant spent in: 1) half smiles; 2) full smiles; and 3) laughter, was used as measures of mirthful duration. Two coders identified the expressions according to a coding sheet of three categories adapted from previous research (Bachorowski, Smoski, and Owren 2001;Olson 1992;Vidulich and Bayley 1966;Deckers, Jenkins, and Gladfelter 1977). The description for category 1 Clear Smile is 'clear upturned corners of the mouth'. Category 2 Big Grin is 'clearly of higher intensity than clear smiles, clear bilateral movement and visible dimples'. Category 3 Laughter is 'any audible obvious sound accompanying either a smile or grin'. The 'laughter' category was coded as 'vocalised' laughter. Eye movements were not included in coding facial expressions as the participants were blindfolded. Mirthful expressions were hand-timed using a stopwatch and measured in seconds (and not 'events').

Statistical data analysis
Inter-observer agreement for all duration measures was high (r s = .9, p < .01), and for peak mirth it was reasonable (Kappa 55%). Average scores were calculated where observers disagreed. There were four dependent variable scores for each participant: 1) highest mirth achieved over the 30 seconds; and average time spent in 2) a half smile; 3) a full smile; and 4) laughing. A series of two-way independent-subjects analysis of variance (ANOVA) were run on each of the four variables, with induced-mood (fear, happiness, neutral) and familiarity (known, unknown) as the two betweensubjects factors. Post-hoc tests were performed using the Bonferonni method. The effect sizes were calculated by hand using the mean difference equation (ES = X i -X j ).

Results
Participant demographic details and group compositions are above. Mean age, gender, and familiarity of participants, did not significantly differ between the three experimental groups. No participants recognised the musical part of the excerpts presented. Self-reported emotions were consistent with intended mood induced in each of the three conditions; negative and arousing emotions in the fearful aroused condition, positive emotions in the happy condition, mostly relaxing, though also some reports of positive and negative emotions in the control condition. Some participants reported that the laughter made them feel uncomfortable. Facial expressions of participants were generally reliably categorized on the mirth scale by observers. Observers expressed particular difficultly when trying to classify faces of participants who appeared to be stopping themselves laugh, where participants were seen to flare nostrils, bite lips, raise eyebrows and fidget or display other behaviours not enlisted in the mirth scale.

Discussion
The present study asked whether laughter produced in the context of negative emotions was modulated by social context. The main finding was a significant affect-familiarity interaction in three of the four dependent variables. In the fearful affect condition participants familiar to the experimenter had significantly greater peak mirth scores, spent longer smiling (half and fully) in response to the infectious laughter than participants unfamiliar to the experimenter. Familiar participants also laughed for longer than unfamiliar participants in the fearful affect condition, but the familiarity-affect interaction was not significant. This is perhaps because fewer participants laughed overall in this condition, making the interaction underpowered.
This experiment was designed to combine previous work showing that laughter is both sensitive to social group and can regulate affective states within interactions. Laughter is greater amongst members of a group (Smoski and Bachorowski 2003) and is increased when participants hear in-group laughter (Platow et al. 2005) or when further affiliation with one's experimental partner is desired (Grammer and Eibl-Eibesfeldt 1990). Additionally, positive affect, including smiles and laughter, down-regulate negative arousal arising from stressful events. This has been demonstrated in stressful events ranging from a forced speech task (Fredrickson et al. 2000), a fearful film Levenson 1998), viewing unpleasant pictures (Sokhadze 2007). The findings presented here uniquely demonstrate that mirthful expression following a transition from negative to positive arousal is greater when in company of in-compared to out-group, members.
Although it was theorised that the significant interaction in the fearful affect condition was driven by more laughter amongst participants who knew the experimenter, it could be driven by less laughter in participants unknown to the experimenter. Graphical representation data of the dependent variables suggest known participants express more amusement in the fearful condition. However, some participants reported that the laughter made them uncomfortable. Laughter itself is a universal laughter-provoking stimulus (Provine 1992) reported by listeners to induce in them a positive affect (Szameitat et al., 2009). However, it can be heard in a negative way, especially in people who are "gelatophobic" and fear being laughed at (Ruch, Hofmann, and Platt 2015;Ruch et al. 2014). Perhaps the unfamiliar participants, who were blindfolded and in the company of a stranger, were more likely to perceive laughter negatively in the fearful affect condition.
Previous work suggests that laughter is greater amongst members of a group compared to strangers (Smoski and Bachorowski 2003;Platow et al. 2005;Grammer and Eibl-Eibesfeldt 1990). In this experiment, however, time spent fully smiling was the only dependent with a significant effect of familiarity; familiar participants fully smiled more than the unfamiliar participants. Familiar company only increased the duration of half smiles and extent of mirth in the context of negative emotions. Similarly, although laughter has been found to accompany the transition from negative-to-positive affect (Fredrickson et al. 2000;Fredrickson and Levenson 1998;Sokhadze 2007;Sroufe and Wunsch 1972), there was no significant effect of "affect" alone. These results suggest that the strongest stimulus to laugh and express is when participants are amongst familiar company and experience a transition from a fearful to positive affective state. This is a stronger stimulus to laugh that just being in familiar company or just experiencing transition from a negative-to-positive affect.
Humour is involved in the formation, maintenance, and regulation of close interpersonal relationships (Lefcourt 2001;Shiota et al. 2004). Interactions associated with laughter increase feelings of closeness and attraction to ones' confederate more than do equally enjoyable but non-humorous experiences (Fraley and Aron 2004). Positive emotions and laughter in humorous communications reinforce mutual feelings of affection, strengthen attachment, and contribute to greater relationship satisfaction (Smoski and Bachorowski 2003). Couples in highly satisfied marriages are better able to regulate negatively aroused states during conflictive interactions using positive affect, including laughter, affection and humour, than couples in less satisfied marriages (Gottman and Levenson 1992;Levenson and Gottman 1983;Carstensen, Gottman, and Levenson 1995) and have greater degrees of marital stability six years later (Gottman et al. 1998). Instructions to suppress negative emotions when discussing negative topics disrupted communication, inhibited relationship formation, and increased the blood pressure of the suppressor's partner (Butler et al. 2003). It follows that laughter, a communicative group-sensitive behaviour, may have functional and evolutionary role in downregulating negative affective states in familiar company to facilitate successful group cohesion and group bonding. There is likewise less functional a role for laughter amongst strangers when faced with stressful situations.
The limitations of this experiment include the use of a blindfold. The involvement of eyes in smiles is important to distinguish between Duchenne ("emotional") and non-Duchenne ("social") smiles when coding smiles in social interaction (Ekman, Davidson, and Friesen 1990). It possible that the smiles recorded in this experiment were both "social" and "emotional" smiles. However, since participants were not directly interacting socially with a companion, "social" smiles were thought to be negligible. Participants were positioned in-front of a camera which could make participants feel self-conscious, and thereby inhibit emotional expressions.
Participants were blindfolded to both aid concentration on acoustic stimuli, and to block out the distraction of a camera. The benefits of using a blindfold for these purposes was thought to outweigh the benefit of including the eyes in rating of degrees of mirth. Another difficulty was found when coding faces for amusement in participants who appeared to be withholding laughter, and often used facial muscles other than the mouth. Future studies may benefit from including an additional "withholding laughter" category on mirth coding scales. "Familiarity" was coded categorically as unfamiliar or familiar, but it is recognized that this variable varies continuously. Future experiments could focus on whether the degree of familiarity influences the degree of mitigation from negatively arousing states. A strength of this experimental design was the use naturalistic methodology to evoke laughter, a sensitive emotion notoriously difficult to evoke in experimental situations. The design also included induction of a positive affective state which demonstrated the humorous response was specific to negative arousal as opposed to arousal in general as has been theorized (Berlyne 1972).
One interesting alternative explanation to the findings in this experiment relates to the 'relief' hypothesis of laughter (Ramachandran, Blakeslee, and Sacks 1998;Hayworth 1928;Keltner and Bonanno 1997). This hypothesis states that laughter accompanies the elicitation of positive affect, namely relief, following a negative state. The relief hypothesis can also provide unifying framework explaining why the focus of jokes worldwide tends to centre of taboo topics and slapstick (Freud 1989;Kotthoff 2006;Gervais and Wilson 2005). Future experiments will need to tease out these two alternative explanations of laughter.

Conclusion
In summary mirth intensity, smiles (half and full) were greater in known compared to unknown participants following induction of a fearful affect. This positive affect can be used to regulate negative affective states amongst people known to each other. The stimulus to laugh may be highest when in familiar companies and following a negatively arousal situation.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
The benefits of publishing with F1000Research: Your article is published within days, with no editorial bias • You can publish traditional articles, null/negative results, case reports, data notes and more • The peer review process is transparent and collaborative • Your article is indexed in PubMed after passing peer review • Dedicated customer support at every stage • For pre-submission enquiries, contact research@f1000.com