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5,200 fruit fly generations) and then hot for another century, only to be cold again in the third century. Natural selection would not know who (with which traits) to select.
    Since the advent of agriculture about ten thousand years ago and the birth of human civilization which followed, humans have not had a stable environment against which natural selection can operate. For example, a mere two centuries (ten generations) ago, the United States and the rest of the Western world were largely agrarian; most people were farmers. In the agrarian society, men achieved higher status by being the best farmers; those who possessed certain traits that made them good farmers had higher status and thus greater reproductive success than others who didn’t possess such traits.
    Then, only a century later, the United States and Europe were predominantly industrial societies; most men made their living working for factories. Traits that make men good factory workers (or, better yet, factory owners ) may or may not be the same as the traits that make them good farmers. Certain traits—such as intelligence, diligence, and sociability—probably remain important, 31 but others—such as a feel for nature, the soil, and animals, and the ability to work outdoors or forecast weather—cease to be important, and other traits—such as punctuality, the ability to follow instructions, a feel for machinery or mechanical aptitude, and the ability to work indoors —suddenly become important.
    Now we are in a post-industrial society, where most people work neither as farmers nor factory workers but in the ser vice industry. Computers and other electronic devices become important, and an entirely new set of traits is necessary to be successful. Bill Gates and Sir Richard Branson (and other successful men of today) may not have made particularly successful farmers or factory workers. All of these dramatic changes happened within ten generations, and there is no telling what the next century will bring and what traits will be necessary to be successful in the twenty-second century. We live in an unstable, ever-changing environment, and have done so for about ten thousand years.
    For hundreds of thousands of years before that, our ancestors lived as hunter-gatherers on the African savanna, in a stable, unchanging environment to which natural selection could respond. That is why all humans today have traits that would have made them good hunter-gatherers in Africa—men’s great spatiovisual skills, which allowed them to follow animals on a hunting trip for days and for miles without a map or a global satellite positioning device and return home safely; and women’s great object location memory, which allowed them to remember where fruit trees and bushes were and return there every season to harvest, once again without maps or permanent landmarks.
    For the last ten thousand years or so, however, our environment has been changing too rapidly for evolution to catch up. Evolution cannot work against moving targets. That’s why humans have not evolved in any predictable direction since about ten thousand years ago. We hasten to add that certain features of our environment have remained the same—we have always had to get along with other humans, and we have always had to find and keep our mates—so certain traits, like sociability or physical attractiveness, have always been favored by natural and sexual selection. But other features of our environment have changed too rapidly relative to our generation time, in a relatively random fashion—who could have predicted computers and the Internet a century ago?—so we have not been able to adapt and evolve against the constantly moving target of the environment.

2
Why Are Men and Women So Different?
    Much of our discussion in the following chapters hinges on differences between men and women. Now everyone knows that men and women are different. On the whole,