**Abstract**

This chapter delves into the topic of contagious laughter, as a little-studied vocalization (with the exception of the pioneering research by Provine), which contributes to the creation, maintenance, and strengthening of social ties. The text offers empirical evidence and arguments that support the thesis that contagious laughter and the laughter provoked by it involve a set of distinctive acoustic and perceptual characteristics and vocal, emotional, and affective effects of possible innate nature. The development of the chapter is also important for offering indirect support to the hypothesis or theories, such as (a) a supposed cerebral mechanism of perception/production of contagious laughter, (b) emotional contagion through vocalizations, and (c) innate behavioral sequences, proposed by the ethological perspective. Based on the reported evidence that supports the formulated thesis, the corresponding theoretical relationships and implications are established. At the end of the chapter, the possible relationships between contagious laughter and happiness are established, as phenomena of phylogenetically ancient origin and related to innate tendencies of positive valence.

**Keywords:** contagious laughter, positive emotions, emotional contagion, emotional expression, innate, happiness

## **1. Introduction**

The order of this chapter is as follows: first, empirical evidence and arguments about the innate nature of contagious laughter are offered, and then an attempt is made to establish a possible relationship between it and the phylogenetic antecedents that happiness would have, some of which could have common elements with those of this type of laughter [1–4].

Although there may be acoustic and facial expression variations in laughter, in general, laughter is a distinctive and universally recognizable emotional expression [5]; it

has been shown that a wide range of laugh syllable repetition intervals is unambiguously perceived as episodes of this vocalization [6, 7]; in fact, laughter is the only positive emotional vocal expression recognized in all cultural groups and in all latitudes [8].

Laughter can be considered a multimodal activity that is basically expressed through facial expressions, vocalizations, and body movements [9]. The facial expression of laughter, when expressing positive emotions, involves involuntary facial movements of the zygomaticus major and orbicularis oculi muscles [10] and elongation and/or elevation of the corners of the lips, which may or may not be accompanied by mouthopening [11]. Laughter, as a vocalization, implies the joint participation of the respiratory and subglottic vocal systems; laughter vocalizations include repetitive periods of laughter syllables that occur within a different temporal structure than speech [12]. The body movements that accompany laughter are rhythmic movements of the head, shoulders, and torso that occur during an episode of this vocalization, the amplitude of which varies according to the intensity of the emotion that it provokes [13].

The acoustic study of laughter involves quantitatively determining the following factors: (a) tone: which refers to how high or low this vocalization is perceived. Within this attribute, the fundamental frequency (FO) has special importance; this is understood as the number of times the vocal cords vibrate in a unit of time [14] and there are several acoustic parameters related to them: range of FO, slope of FO, standard deviation of FO on duration of the laughter stimulus [15]; (b) intensity: refers to the acoustic energy with which this vocalization is produced [16]; (c) timbre: it has been defined as the specific qualitative aspects that distinguish each laugh [17]; (c) duration of the different acoustic components of laughter (periods, syllables, episodes and intervals between them) [16] and (e) spectral variables: attributes related to the spectrum of sinusoidal waves in which the sound of laughter can be broken down [18].

Acoustic analysis of laughter can be performed based on laughter episodes, periods, syllables, and segments. The set of laughter periods is called the laughter episode; the periods refer to each of the laughter events produced during an exhalation [19]; each of the sound parts of these periods is called laugh syllables [20] and the laugh segments are the components of this vocalization temporally delimited in the spectrogram [19].

Laughter is a vocalization that the human being shares with other animal species, especially with nonhuman primates [21], whose laughter differs from the human in the syllabic structures and in the respiratory and acoustic patterns of its production, although both share the brain areas that control them [20] and the adaptive functions of this vocal expression: positive emotional expression and improvement of social ties [10].

Although there are different types of laughter, each with particular brain correlates and distinctive facial and acoustic expressions, the main classification of laughter is that which distinguishes between voluntary laughter, or non-Duchenne laughter, and involuntary laughter, or Duchenne laughter, each with a different neural pathway [10]. This distinction is important because it indicates the two great functions that have been attributed to laughter, which can occur jointly or independently: to express emotions spontaneously and non-verbally and to serve as a sign of acceptance or welcome that facilitates social interactions [5].

Duchenne or authentic laughter is a laughter elicited by stimuli that generate positive emotions and a distinctive facial expression; it originates in subcortical regions: amygdala, thalamic and hypothalamic areas, and the dorsal-tegmental areas of the brain stem [22]; human beings have a genetic predisposition to develop this type of laughter from very early ages [10]. In contrast, voluntary laughter or non-Duchenne

*Contagious Laughter as an Innate Acoustic Stimulus That Provokes Positive Emotions... DOI: http://dx.doi.org/10.5772/intechopen.108336*

laughter is not associated with emotional experience, does not have a distinctive facial expression, and has different acoustic characteristics and brain correlates than authentic laughter [7]; it generally indicates a desire for affiliation, the intention to appease the other or a courteous agreement with the interlocutor, although it can also be emitted with the intention of mocking or attacking others [10].

Within Duchenne laughter, it could locate a subtype of laughter that has received the name of contagious laughter, which, unlike other spontaneous laughter that occurs due to exposure to humorous stimuli, this vocalization is caused by listening to the laughter of other people [1]. Taking this type of laughter as a common thread, this chapter aims to offer empirical evidence and arguments to prove the thesis that contagious laughter and the laughter provoked by it involve a set of acoustic characteristics, affects, emotions, and behavioral patterns that human beings innately bring, which facilitate social interaction with congeners.

The verification of this thesis has both theoretical and practical importance since the pioneering study by Provine [1] on this subject formulated hypotheses about the innate nature of contagious laughter, which have not been verified to date; on the other hand, the verification of the innate nature of this laughter and the laughter/ smiles caused by it could shed light on the mechanisms of innate positive social influence, which would enrich the possibilities of practical applications in different social and/or psychological areas.

### **2. Contagious laughter and the laughter caused by it**

Contagious laughter is such a common phenomenon in everyday interactions that for a long time it was not considered an important or interesting object of scientific study. However, Provine [1] saw in the study of this laughter an opportunity to examine one of the typical responses of the human species. For this author, contagious laughter is a liberating stimulus of this same behavior in another person or persons, for which it assumes the characteristics of a typical stereotyped response of the species that would have evolved to facilitate social synchronization [23]. In this same sense, other authors consider that the function of contagious laughter is to facilitate social interaction [2] and the achievement of collective goals [1].

The definition that Provine proposes of contagious laughter as a liberating stimulus indicates the ethological perspective from which this type of vocalization was initially studied. From this perspective and based on the conceptualization of Gómez and Colmenares [4], contagious laughter could be considered an "innate liberating mechanism" that would connect the perception of the sign stimulus with the effector organs of the elicited behavior and the provoked laughter would be a modal action pattern, which would admit a certain degree of variability both at the level of the individual and the species.

Based on the ethological perspective, Provine [1, 24, 25] hypothesizes that through evolution, human beings have developed an innate mechanism detecting contagious laughter that perceives its distinctive acoustic attributes that would automatically trigger the motor pattern of laughter in listeners. This supposed mechanism that would relate to the production and perception of contagious laughter suggests a functional relationship or a parallel evolution between both functions.

In addition to the supposed neural mechanism involved in contagious laughter proposed by Provine [24, 25], which, if verified, would confirm the innate nature of this vocalization, other authors provide direct or indirect evidence on it: Davila-Ross et al. [26] point out the possible phylogenetic antecedents of this laughter in nonhuman primate species; Nwokah et al. [27] found evidence on the appearance of this laughter from early stages of human ontogenetic development since they observed that infants under five months present laughter with a latency of less than or equal to 4 seconds and of the same duration as the laughter expressed by the mother; Bard [28] points out that nonhuman primates can replicate the facial and vocal expressions of their conspecifics (including expressions of laughter) with a duration of less than one second to improve affiliation and coordination of activities.

In his original study, Provine [1] verified the existence of contagious laughter and its characteristics. Provine's attempted to test the hypothesis that exposure to exclusively acoustic laugh stimuli would induce laughing or smiling responses in participants. The author took a sample of 128 university students, who listened to 10 contagious laugh audios (taken from laugh-inducing commercial boxes) for 18 seconds. Faced with these stimuli, a little more than half of these subjects reported having experienced laughter or smiles in the first trials, responses that were less frequent in the later trials. On the other hand, he observed that the provoked laughter replicated the motor pattern of the original vocalization and a parallel contagion of the physiological state of the emitter of laughter: respiratory pattern, cardiovascular and autonomic responses, which would suggest that contagious laughter not only implies a social and behavioral synchronization but also a coupling of the physiological state between the members involved in the phenomenon.

Methodologically, Provine's study [1] is weak due to (a) the stimuli used that were not acoustically characterized, (b) the use of self-report to establish the responses of laughter or smile in the participants, and (c) the nonuse of objective measures to assess other physiological responses that could cause these stimuli. Unfortunately, the literature does not report more empirical studies on contagious laughter, with the exception of those reported in this chapter, which tried to overcome the methodological deficiencies of the original study of this type of laughter.

### **2.1 Appreciation of contagion and acoustic parameters of contagious laughter**

Within a perspective that considers contagious laughter as a liberating stimulus, it is relevant to identify the differential acoustic characteristics of this vocalization that have the supposed capacity to provoke the perception of contagion and the response of laughter or smile in other people; however, it was not possible to find a study on this topic. Given this lack of knowledge, the research by Arévalo-Pachón & Cruz [29] described and related the acoustic parameters of more and less contagious laughter stimuli and the perception of their contagion.

The research by Arévalo-Pachón and Cruz [29] selected a sample of 66 acoustic stimuli of adult laughter classified as contagious (33 of male laughter and 33 of female laughter). A third of the male and female laughter stimuli lasted between 4 and 7 seconds; the other, between 8 and 11 seconds, and the last, between 12 and 16 seconds. Laughter stimuli were selected from videos of contagious laughter, approved as such by laughter experts, acoustically cleansed (with Audicity App, version 2.2.2, https:// www.audacityteam.org/download/), and recorded in WAV format. Laughter stimuli were randomly presented to participants using the program Qualtrics (Qualtrics Provo, UT, versión 2018), which allowed applying the online test to a large number of participants. On the other hand, a convenience sample of 132 university students (84 women, 48 men) under 30 years of age and with normal hearing and vision from the city of Bogotá was taken.

### *Contagious Laughter as an Innate Acoustic Stimulus That Provokes Positive Emotions... DOI: http://dx.doi.org/10.5772/intechopen.108336*

In the first part of the study, the subjects listened to the 66 audios of laughter with headphones and had to rate them according to their appreciation of contagion on a scale of 0 to 10 (0 being no contagion, and 10 being maximum perceived contagion). In the second part of the study, the 66 audios of contagious laughter were acoustically characterized using the PRAAT software, version 6.0.3.7 (https://Praat.uptodown. com/windows); the acoustic parameters of all the laughter stimuli were established based on a sampling frequency of 44,100 Hz and with a pitch range of 75 to 550 Hz for women and 75 to 500 Hz for men [16]. Following the recommendations of Wood et al. [15] and Correa (16], about the acoustic parameters to be selected and how to determine their values, 12 acoustic parameters were selected: total duration (s), duration of laughter periods (s), average intensity in the laughter simple (db), average FO, range of F0 (Hz), standard deviation F0/duration (Hz/s), average of the slope of F0 (Hz/s), center of gravity (Hz), average harmonics/noise ratio (Db), average formant F1 (Hz), average formant F2 (Hz), and proportion of unvoiced segments (%).

In this study, it was confirmed that there are male and female laughter stimuli perceived as more contagious than others. Thus, significant differences were observed between the three most contagious female and male laughs (M = 669.65) and the three perceived as less contagious (M = 235.49): [t = 11.88(5) p < 0.05]. Significant differences were also observed between the three female laughs that were more (M = 645.47) and less contagious (M = 238.96): [t = 7.33 (2) p < 0.05] and between the three male laughs perceived as more (ME = 693.84) and less contagious (M = 232.02): [t = 8.93 (2) p < 0.05]. These results confirm the ability of humans to discriminate and qualify the level of contagion of acoustic stimuli of laughter.

Although the three female laughter stimuli qualified as the most contagious present an intensity average higher than the average of this parameter in all the female laughter evaluated, this difference is not statistically significant; on the other hand, the female laugh stimulus perceived as more contagious has the highest value of the proportion of non-voiced segments among all female laughs. As for the three male laughter stimuli qualified as the most contagious, these present average values in duration of laughter periods, average of F0, and an average of the slope of F0 higher than the average of all the male laughter stimuli evaluated. The only one of these parameters that showed significant differences was duration of laughter periods (average time from start to finish of laugh sections separated by inspirations, [30]) (M = 3,68, M = 25,0, respectively) [t = −8.44(2) p < 0.05]. When the three female laughter stimuli rated as more contagious are compared with the three female laughter rated as less contagious, it is observed that the only attribute that shows significant differences between both groups is: the standard deviation of F0/duration (captures moment-to-moment variability in pitch) (M: 6.71 and M: 8.43, respectively): [t = −4.76 (2) p < 0.05]. On the other hand, the comparison of the 3 masculine laughter stimuli qualified as more contagious with the three masculine stimuli evaluated as less contagious, allows us to observe that the only attribute that shows significant differences between both groups is duration of periods of laughter (M: 3.68 and M: 1.91, respectively): [t = 6.11 (2) p < 0.05]. These results indicate the existence of some acoustic parameters that would characterize the laughter as more contagious or less contagious, which would induce a different laughter response in listeners.

One of the hypotheses that was tested in this research affirmed that "the values of the acoustic parameters that characterize the stimuli of contagious laughter allow to predict the qualifications of appreciation of contagion of these." This hypothesis was statistically confirmed by means of multiple stepwise regression (dependent variable: contagion scores and predictor variables: average Fo, average range of Fo,

total duration, duration of laughter periods, harmonic ratio/noise, spectral center of gravity, proportion of non-voiced segments, average of F1, average of F2, and intensity). The assumptions of the multiple regression were met satisfactorily. The multiple regression model (with N = 132) that was more predictive and significant included the acoustic parameters mean duration of laughter periods (defined in previous paragraphs), mean F1 (points out the influence of the first acoustic resonator on the pitch of laughter [31]) and mean F2 (point out the influence of the second acoustic resonator on the tone of laughter [31]), whose statistical values were: R2 corrected: 0.188 F = 6.029 (3, 62) p = <0.05. According to the β values of the regressors, the acoustic parameter most predictive of contagion is the average duration of the laughter periods; followed by mean of formant 2 (F2) and mean of formant 1 (F1). This result would confirm the ability of some specific acoustic parameters to explain the perception of contagion caused by some laughs.

As multiple regression analysis shows the duration of the periods of laughter is the predictor of contagion perception with the greatest weight, which suggests that only the prolonged repetition of laughter syllables and other audible and non-voiced elements emitted within the same expiration would largely determine the judgments of appreciation of contagion in the listener. Given that longer periods of laughter are difficult to fake or produce voluntarily [32], this attribute would indicate the spontaneous nature that this laughter must have in order to be perceived as contagious. These results confirm the finding of research by Neves et al. [33] that found that the subjects who report contagion perception in stimuli of laughter tend to be based on the detection of the authenticity of this vocalization. Taking into account that the production and perception of spontaneous laughter are regulated by phylogenetically ancient brain mechanisms [34], this finding suggests the innate character of this type of laughter.

The regression model also includes high values of F1 and F2 as key components for the perception of contagion of this laughter: the high values of F1 suggest that this vocalization is perceived as produced with open vowels [31]; the high values of F2 suggest the positivity with which this laughter is perceived [35] and the high values of F1 and F2 taken together, indicate that this laughter is perceived as a vocalization with a particular and distinctive timbre [17].

In summary, the results of this study indicate that contagious laughter would have distinctive acoustic and perceptual characteristics, which would be related. The relationship between these variables suggests biological preparation and evolutionary molding produce and perceive this vocalization.
