Original ArticleSharing the joke: the size of natural laughter groups
Introduction
Although by no means unique to humans (it occurs in great apes: Davila-Ross et al., 2009, Waller and Dunbar, 2005), laughter is one of the most distinctively human behaviors (Gervais and Wilson, 2005, Provine, 2001). While a number of (not necessarily mutually exclusive) hypotheses have been suggested for its function (signaling social or mating interest: Grammer, 1990, Grammer and Eibl-Eibesfeldt, 1990, Li et al., 2009, Martin and Gray, 1996, Mehu and Dunbar, 2008; emotional contagion: Bachorowski and Owren, 2001, Owren and Bachorowski, 2003; social bonding: Dunbar, 2004, Dunbar et al., 2012), laughter in humans is characteristically highly social and intensely contagious (Provine, 2001). The occurrence of laughter during an interaction also significantly increases the perceived satisfaction with the interaction (Vlahovic, Roberts, & Dunbar, 2012).
Anthropoid primates are characterized by an unusually intense form of social bonding (Dunbar and Shultz, 2010, Shultz and Dunbar, 2010) that is mediated by an endorphin-based psychopharmacological mechanism effected by social grooming (Curley and Keverne, 2005, Depue et al., 2005, Machin and Dunbar, 2011). Social grooming (the bimanual cleaning and manipulation of a recipient's skin or fur) is limited to dyads since it is physically difficult to groom several individuals at the same time. Given this, its effective broadcast group size (the number of individuals whose state of arousal can be influenced in this way) is one. This, combined with limits on the time available for social grooming (Dunbar et al., 2009, Lehmann et al., 2007), seems to set an upper limit on the size of social group (or community) that can be bonded through this mechanism (Dunbar, 1993).
Laughter is known to release endorphins in much the same way as grooming does (Dunbar et al., 2012), and this has led to the suggestion that the exaggerated forms of laughter characteristic of humans might have evolved out of conventional ape laughter (Davila-Ross et al., 2009, Davila-Ross et al., 2011) as a device for enlarging the effective size of grooming groups through a form of “grooming-at-a-distance” (Dunbar, 2012). When hominins evolved larger social communities than those characteristic of the most social monkeys and apes, some additional mechanism was required to make this possible. Increasing grooming time was not an option because it was already at its upper limit in primates (Dunbar, 1993, Dunbar et al., 2009), but increasing the number of individuals who could be “groomed” simultaneously is a plausible alternative. Laughter as a form of chorusing (sensu Burt and Vehrencamp, 2005, Schel and Zuberbühler, 2012, Tenaza, 1976) seems to fill that role admirably because it allows several individuals to be involved simultaneously. The fact that human laughter shares close structural similarities with ape laughter (Davila-Ross et al., 2009, Provine, 2001) suggests that, if it was the solution to this problem, it may have been an early adaptation, long predating the evolution of speech and language (Dunbar, 2009, Dunbar, 2012).
This suggestion raises the question of laughter's efficiency as a bonding mechanism relative to social grooming. Given that grooming has an effective broadcast group size of one, just how large is the broadcast group size for laughter? To determine this, we observed natural social groups in bars and collected data on the number of people who laughed together within these groups. We also sampled the size of the whole social group as well as the size of conversational groups (the number of people engaged in a conversation) to provide benchmark measures that enable comparisons between laughter and conversation (conversation groups are known to have an upper limit of four individuals, irrespective of the size of the social group: Dunbar, Duncan, & Nettle, 1995).
Section snippets
Method
We censused natural social groups in bars in the United Kingdom (Oxford; 80% of the observations), France (Calais, Lille, and Paris; 14%), and Germany (Berlin; 6%), distinguishing social group size (the total number of individuals present as an interacting group), conversational subgroup size (the number of individuals within the social group taking part in a particular conversation, as evidenced by speaking or obviously attending to the speaker, following Dunbar et al., 1995), and laughter
Results
Fig. 1 plots the frequency distribution of social, conversational, and laughter subgroup sizes. Average conversation subgroup size was 2.93±0.05 S.E. (N= 501), and average laughter subgroup size was 2.72±0.04 S.E. (N= 501). Conversational subgroups larger than 5 were rare (2.8% of the observations), and none were larger than 10. Similarly, laughter subgroups larger than four were rare (5.6% of the observations), and none were larger than six. Overall, approximately 91% of all conversational
Discussion
Our results confirm, with a considerably larger sample, the upper limit of N≈ 4 on conversation group size reported by Dunbar et al. (1995). In addition, they suggest that there is a similar limit on the number of individuals that can be involved in a laughter event. Ours was, of course, a naturalistic study and thus benefits by all the advantages of ecological validity that this offers. While it might have been possible to run the study in the laboratory with convened groups of predetermined
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