There is no one way of having sex we “evolved” for—we are flexible sexual and social strategists. But our essence, if we can be said to have one, is likely less matron and more macaque. Female infidelity is a behavior with one foot in the present day and the other in our ancient past, linked to anatomy, physiology, and reward seeking. And the best mother is the one who, when circumstances are right, does what it takes to line up allies who will be well disposed toward her offspring. She might do so on her back, or with her rump in the air.
