It’s possible to fall in love with someone just because you had sex with them, because with orgasm you get a flood of oxytocin and vasopressin that can cause you to feel attached to the person. Casual sex is not always casual. Someone might be happy, with a lovely wife or husband, and then this brain system for romantic love or attachment is triggered by someone totally inappropriate. You acquire all the characteristics of an addict. You come obsessed; you distort reality; you do dangerous things; you crave the person, you have withdrawal symptoms.

How to avoid that fate when, say, you find yourself tempted to have a fling at a convention? “Take enough Prozac beforehand,” Dr. Fisher said, “and your emotions will be so blunted that you won’t even get into bed with anyone.”