Philosopher John Morreall believes that the first human laughter may have begun as a gesture of shared relief at the passing of danger. And since the relaxation that results from a bout of laughter inhibits the biological fight-or-flight response, laughter may indicate trust in one's companions.
Many researchers believe that the purpose of laughter is related to making and strengthening human connections. "Laughter occurs when people are comfortable with one another, when they feel open and free. And the more laughter [there is], the more bonding [occurs] within the group," says cultural anthropologist Mahadev Apte. This feedback "loop" of bonding-laughter-more bonding, combined with the common desire not to be singled out from the group, may be another reason why laughter is often contagious.