PARENTS, TEACHERS, HEALTH professionals, and others concerned about the issues surrounding sexually active teenagers usually begin by asking two seemingly simple questions: First, how many teens are having sex? Second, why did these young people become sexually active in the first place? Answering the first question, they hope, will reveal the scope of the problem; answering the second question may provide insights about how to deal with the problem.

Researchers who study sexual behavior invariably report that these and other questions about teenage sexuality are far from simple to answer. Collecting accurate and meaningful statistics about sex, including teen sex, is difficult. Most surveys and studies include a few thousand teenagers at best, groups, some critics argue, which may not reliably reflect the behaviors and attitudes of society at large.

Even when...