"Many public-policy decisions are based on implicit assumptions about 'human nature,' and it is currently popular to speculate about how evolution might have shaped human behavior and psychology. But this raises some important questions: are humans continuing to evolve—and, if so, is our basic biological nature changing—or has modern culture stopped evolution? For some traits, we do not have to speculate—we can measure and compare on the basis of studies covering thousands of individuals over several generations. Such studies have not yet been done on most of the traits where speculation is popular, but they have been done on some traits of medical interest. What have we learned?"