J. Acoust. Soc. Am. 124, 472–483 (2008)

Credit: B. FASANI/CORBIS

Laughter is considered to be a reflex action, an adaptive tension-reliever with analogues in many non-human species. That congenitally deaf people laugh out loud supports this theory. But do they produce the same sounds as those who hear normally?

Maja Makagon of Cornell University in Ithaca, New York, and her colleagues showed congenitally deaf volunteers clips of films such as Mr Bean and The Naked Gun, and compared the acoustic properties of their laughter with that of unimpaired controls. The quality of the sound was remarkably similar; the differences in the sound-waves' shapes were more consistent with deaf people having less vocal-muscle control than with hearers having learned how laughing 'should sound'.

The deaf volunteers laughed more quietly, perhaps owing to social conditioning that led them to lower vocal volume overall.