The evolutionary roots of laughter

To evolutionary psychologists, the noise made by gorillas, chimps and bonobos when you tickle their feet is no laughing matter. These distinctive vocalisations suggest that rather than evolving separately, laughter evolved in a shared common ancestor before becoming tailored in each primate species, including humans.

To find support for this idea, Diana Szameitat and her colleagues scanned the brains of 18 men and women whilst they listened to the sound of human tickle-induced laughter as well as laughter prompted by joy and taunting. The researchers found a 'double-dissociation' - the tickle laughter provoked extra activity in the secondary auditory cortex, likely reflecting the acoustical complexity of this kind of laughter, whereas the joy and taunting laughter prompted more activity in the medial frontal cortex, a region associated with social and emotional processing. These differences were observed whether the participants were tasked with categorising the laughter they heard, or merely with counting the number of laughs. The finding suggests that humans produce and process an evolutionarily 'old' form of tickle-based laughter, which is shared with non-human primates, as well as a newer, more emotionally sophisticated variant.

The laughter stimuli were provided by a team of eight professional actors using 'auto induction' techniques. This means they used their imagination, memories, and body movements to provoke the required emotions and bodily sensations in themselves as far as they could. The researchers said they only selected laughter samples that had been accurately categorised (as joy, taunting, or tickle laughter) in pilot work at well above chance levels by naive listeners. The dependence on acted laughter does seem to be a weakness of the study, however, especially as it's a well-documented fact that people are unable to tickle themselves.

'Our study provides suggestive evidence that laughter, in the form of a reflex-like reaction to touch, has been adopted into human social behaviour from animal behaviour,' the researchers said. 'Through the differentiation of human social interaction over time this "simple" form of laughter may have diversified to become a spectrum of different laughter variants in order to accommodate increased complexity of human social interaction.'
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Szameitat, D., Kreifelts, B., Alter, K., Szameitat, A., Sterr, A., Grodd, W., and Wildgruber, D. (2010). It is not always tickling: Distinct cerebral responses during perception of different laughter types. NeuroImage, 53 (4), 1264-1271 DOI: 10.1016/j.neuroimage.2010.06.028