All laughs are not created equal in listeners' ears. New research shows that people respond the most positively to vowel-like bursts issued from an open mouth – the types of loud "ha-has" heard on the laugh tracks of sitcoms or from Sesame Street's Elmo. And the longer this laughter continues, the more listeners seem to like it.

The least contagious laugh, by contrast, is a breathy one that does not vibrate the vocal cords, according to two laughter researchers, Michael Orwen of Georgia State University and Tobias Riede of the University of Utah. (Think, those courtesy laughs for your boss's sake.)

In one experiment, 28 college students rated 48 different bouts of "voiced" laughter – half open-mouth and half closed-mouth laughs. All of the laughter was voiced — that is, it vibrated the laugher's vocal cords.

The students rated all of the laughter as positive, but they gave higher ratings to the laughter from the open mouths. In another experiment, listeners responded more positively to longer bouts of open-mouthed laughter than shorter stints, according to the researchers.

Previous work had shown people prefer voiced guffaws to unvoiced, such as a snorting laugh made through the nose.

All of the laughter used in this research was recorded from people watching clips of the film comedies "When Harry Met Sally" or "Monty Python and the Holy Grail" or from playing games, Orwen said.   

"An important point is that the laughter was all associated with circumstances when the laughers were in a positive state," he told LiveScience.

This raises a question: If all of the laughter arose from genuinely happy circumstances, why do listeners uniformly seem to prefer a certain type?

Orwen suspects it may have to do with our ability to detect just how funny the laugher finds something, otherwise known as the level of arousal of their laughter.

Open-mouthed laughter may particularly stand out because as one's mirth increases, the vocal folds are increasingly set in motion and the mouth opens. Unconsciously, listeners may respond to this change, the researchers hypothesized.

To test this, the researchers plan to have participants watch more video clips and record their laughter. But in this forthcoming experiment participants will rate their level of enjoyment from moment to moment, while the researchers record their skin conductance (a measure of emotional response such as stress) and their heart rates.  

Their work will be presented Nov. 19 at the Second Pan-American/Iberian Meeting on Acoustics in Cancun, Mexico.