Identical twins are not necessarily spitting images. Take a set of boy twins separated at birth: one is raised in a nurturing environment and develops into a tall, hearty man, while the other, neglected and malnourished, grows up to be significantly shorter and punier. What more powerful way to demonstrate how the environment can shape human development?
That interaction is at the heart of many of the discoveries described in Men: Evolutionary and Life History, a new book from Richard Bribiescas, the director of the Reproductive Ecology Laboratory at Yale.
What he finds isn't all that comforting. Testosterone, for example, is crucial to survival but possibly poisonous in the long run. Although the hormone bolsters male reproductive effort, it also fosters the growth of prostate cancer. Or consider parenthood: Because of men's comparatively fleeting role in the propagation of the human species, they are not biologically required to invest energy in their offspring or kin. They do not give birth or breast-feed, and they "can't be positive that they fathered particular children," notes Bribiescas.
Worse yet, men, who make up roughly half of the worldwide population, account for 85 percent of the violent crime in the United States and other countries. They are more likely than women to engage in risky or life-threatening behavior. Bribiescas takes a long view of the basis for these actions. He suggests that the association of maleness not only with crime but also with war and independence may ultimately have as much to do with their role as hunters and protectors as with their marginal place in the family unit. "From an evolutionary perspective," Bribiescas concludes, "males are quite alone."