Laughter appears to be a “human universal”: one of those many traits that Donald Brown, in his book Human Universals, found in every society.  Well, does that mean it is a trait that evolved in our ancestors, or did it merely appear as a cultural phenomenon early in human society, and spread to all other societies?  One bit of evidence is that children who are deaf and blind, and thus can’t see or hear other people laughing, still laugh.  This suggests (but of course does not prove) that it is an innate, genetically coded trait, though it says nothing about whether it might have been an adaptive trait.

New research just reported in Current Biology by Davila Ross et al. (see here for a short BBC summary of the work; be sure to click on the video to see a gorilla “laughing”!) suggests that laughter is at least an evolved phenomenon, for our relatives appear to show similar vocalizations when tickled, and our closer relatives show more similar vocalizations.

Davila Ross et al. tickled 3 human infants, 7 orangutas, 5 gorillas, 4 chimpanzees, 5 bonobos (pygmy chimps) and 1 siamang, recording their vocalizations. (What a great job!)  Acoustic analysis of the vocalizations produced a phylogeny, or “family tree” of their similarities.  Strikingly, the family tree based on “tickle-vocalization” analysis is congruent with the known phylogeny based on DNA analysis:

Importantly,  the authors note that several features of human laughter, like its rapid “ha ha ha” type of vocalization, and its expression only during “egressive airflow” (science-ese for “breathing out”) are found in our relatives as well.

So laughter, at least when being tickled, appears to be an evolved, innate phenomenon.  As I emphasized above, this says nothing about whether it was selected for directly, whether it was a byproduct of something else that was selected, or is simply a nonadaptive epiphenomenon.  But as I write, evolutionary psychologists are working on why evolution may have promoted laughter.  Stay tuned.