The great American population explosion may have fizzled, but copulation seems to be more popular than ever. In 1965, the federally funded National Fertility Study, which polled some 5,000 women, reported that the frequency of sexual intercourse for married women under 45 in the U.S. averaged 6.8 times a month. A follow-up study reveals what the country's married couples have been doing since then. On the basis of interviews with 4,000 women in every social and economic class and a comparison of the yearly rates of unwanted pregnancies by couples who do not practice birth control, Charles Westoff of Princeton University's...

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