<p>Laughter is a profoundly social nonlinguistic behavior that functions primarily as a signal of affiliation and a facilitator of social bonding. Some evidence supports the anecdotal observation that laughter is contagious, but little empirical work has measured actual laughter contagion, much less the contextual variables that predict it. In Study 1, 79 participants rated the pleasantness of 93 laughter segments (31 posed laughs, 31 “stranger” laughs, and 31 “friend” laughs) while being videotaped. Two independent raters coded laughter contagion, the presence of participant laughter, in response to the 93 laughs. Friend laughter elicited more contagious laughter than stranger laughter, but friend and posed laughter did not differ. Study 2 (<i>N</i> = 308) attempted to replicate the results of Study 1 using some novel stimuli and additional rater judgments. Perceived authenticity and pleasantness demonstrated the predicted pattern, with friend laughter sounding the most authentic and pleasant, followed by stranger laughter, and then posed laughter. However, these laughter types did not differ by perceived contagiousness. A polynomial regression that regressed behavioral contagion ratings from Study 1 onto perceived authenticity ratings in Study 2 revealed a curvilinear pattern—behavioral contagion was highest for highly inauthentic (posed) laughs and highly authentic laughs. Acoustic analyses suggested that cues associated with greater physiological arousal predicted perceived contagiousness in Study 2. This research is among the first to demonstrate differences in behavioral laughter contagion as a function of laughter type and potentially implicates two unique theories responsible for contagious laughter—benign violation and social bonding/affiliation.</p>