Most of the things that fall on the experience/bodily function area are "contagious". In this sense, yawning, vomiting, and even orgasms are contagious because of a type of cell in the brain called "mirror neurons". A person who may not find something funny may inadvertently get the giggles and eventually believe it's funny through contagion or more specifically, the James - Lange hypothesis. Nurture overtakes nature.