Sex, that great purveyor of arousal, ecstasy, heartbreak, and disease, that dominant theme of poetry, gossip, and entertainment, is on the wane. Surveys suggest that adults are having a lot less of it and high-school students are likely to have had none at all. Theories to explain the shift abound, with responsibility variously placed on social media, increased stress, environmental toxins, sleep deprivation, career pressure, shifting gender roles, attitudes toward sexual harassment, acceptance of masturbation, community isolation, and just about every other feature of contemporary life. What’s up with the libido?