That has two momentous consequences. First, a single male can fertilize several females, forcing other males to go mateless. Males must compete for access to females by beating each other up, cornering the resources necessary to mate, or persuading a female to choose them. That makes females more discriminating in their choice of sexual partners. Humans have added some twists to the mammalian pattern.
Men generally invest in their children by providing food, protection, and care.
Females, like males, may be tempted by infidelity, though their genetic motive is quality rather than quantity. A discreet adulteress can get the genes of the fittest male and the investment of the most generous male. Blum is a superb science reporter who presents just the right amount of complexity, tries to explain findings rather than just report them, and writes in a consistently clear and pleasant style. Sex on the Brain is such a good window on the state of the art that its only flaws are the flaws of the researchers themselves. Unlike Robert Wright and Matt Ridley, who have also written excellent recent books on the biology of sex, Blum does not ground her own story in rigorous evolutionary biology, but rather lets the laboratory scientists speak for themselves.