Sexual conflict permeates ancient religions, from injunctions about thy neighbor’s wife to the permissible rape of infidels. It is etched in written laws that dictate who can and cannot have sex with whom. Its manifestations shape our sexual morality, evoking approving accolades or contemptuous condemnation. It produces sexual double standards that flourish even in the most sexually egalitarian cultures on earth. And although every person alive struggles with sexual conflict, most of us see only the tip of the iceberg: dating deception, a politician’s unsavory sexual grab, the slow crumbling of a once-happy marriage, a romantic breakup that turns nasty. When Men Behave Badly shows that this “battle of the sexes” is deeper and far more pervasive than anyone has recognized, revealing the hidden roots of sexual conflict — roots that originated over deep evolutionary time — which define the sexual psychology we currently carry around in our 3.5-pound brains. Providing novel insights into our minds and behaviors, When Men Behave Badlypresents a unifying new theory of sexual conflict, and offers practical advice for men and women seeking to avoid it.