A glance at any American newspaper published during the late 1990s is enough to convince anyone that the nation is suffering from a collapse of moral values. Accounts of political sex scandals and corruption, parents murdering their children, and ghastly school massacres paint a picture of a society that is morally dissolute, if not outright evil.

On the other hand, recent statistics compiled by the U.S. National Center for Health report encouraging trends in social values, noting that rates of homicide, divorce, illegitimacy, teenage births, and teenage sex have been steadily decreasing since the mid-1990s.

David Whitman, author of The Optimism Gap: The I’m OK–They’re Not Syndrome and the Myth of American Decline, offers one explanation for these conflicting indicators of morality. Although Americans may regard society as morally corrupt, he contends, they usually...