Laughter is a fundamental part of everyday life. Stripped of its local variation and nuance, laughter is a part of the universal human vocabulary, produced and recognized by people of all cultures.     Given the universality of laughter, our ignorance about its purpose and meaning is remarkable. Most people think of laughter as no more than a simple response to a humorous situation, or us a therapeutic mood-lifter. Instead, research on this little-studied topic concludes that laughter is primarily a social vocalization that binds people together. It is not a learned reaction but an instinctive behavior. It is a hidden language that we all speak.   Laughter is contagious, as anyone who has ever giggled at the sight of someone doubled over laughing can attest, Since our laughter is under minimal conscious control, it is spontaneous and largely uncensored. Contagious laughter is a compelling display of Homo sapiens as a social animal. It strips away our outside seriousness and shows we are not in full control of our behavior.   Students in a recent study confirmed the social nature of laughter by recording the circumstances of their laughter in diaries. After excluding the social effects of television and other media, laughter was 30 times more frequent in social than solitary situations. Laughter is a signal we send to others, and it virtually disappears when we lack an audience.   Dose a sense of humor or a lighthearted personality adds years to your life? Not necessarily. Laughter is an energetic activity that raises our heart rate and blood pressure, but these physiological effects are not fully documented, and their medicinal benefits are even less certain. Rather, laughter unites people, and social support has been shown in studies to improve mental and physical health, The presumed health benefits of laughter may be coincidental consequences of its primary goal - bringing people together.