The evidence is accumulating that we have passed through an Age of Liberation and are now in an Age of Restraint. Statistics on sexual behavior, consumption habits and social behavior all show this trend.

*By 1982, the steady increase in female teen-age premarital intercourse had stopped, according to the National Center for Health Statistics. More recently, syphilis is down 30 percent from 1982 -- a vivid contrast with a rise of 50 percent in the preceding five years. One factor, though not necessarily the only one, is fear of genital herpes and AIDS. The change is profound; as one expert put it, "You have to believe that behavior patterns have changed substantially throughout the country."

*The number of abortions leveled off starting in 1981, after roughly doubling from 1973 to 1979, according to the Alan Guttmacher Institute. The abortion rate declined for the first time in 1982. This may have resulted from greater sexual abstinence or from greater revulsion toward abortion, or both.

*The divorce rate declined from 22.8 percent in 1979 to 21.7 percent in 1982, ending a long-term upward trend. The absolute number of divorces also declined.

*Alcohol use is way down. The liquor industry is notoriously in trouble, and beer consumption continues a long-term decline. Even wine sales are languishing.

*Tobacco use among men in 1984 was 35 percent, down from 52 percent in 1964. Tobacco use among women was 29 percent, down from a peak of 34 percent. In 1984 teen-age smoking declined more than it has in 20 years; 19 percent of teens smoked, compared with 29 percent as recently as 1977.

*The statistics on marijuana use aren't very reliable, but they point in one direction: use is down. The National Institute on Drug Abuse says marijuana use among youths 12 to 17 declined from 17 to 12 percent between 1979 and 1982, and use among young adults 18 to 25 declined from 35 to 27 percent. Surveys of college youth find similar trends.

*Crime rates have dropped sharply in the 1980s: the FBI index of major crimes was down 10 percent between 1980 and 1983, and the Justice Department's National Crime Survey found crimes against households down 13 percent in the same period. This happened even though the number of males 15 to 24, who commit most crimes, is down only 4 percent. Prison populations are way up, from 196,000 in 1972 to 463,000 in 1984. Restraint is being exercised: either self-restraint by young men who would have committed criminal acts in years past or restraint by a society increasingly willing to lock up convicted criminals.

Not every trend in our society points in the same direction. Cocaine use is probably up over the past few years, and we have more single-parent families than ever before. But these are trends that result mainly from single segments of the population. The trends I've cited touch the large majority of adult Americans. They result not from the pronouncements of a few politicians or intellectuals but from the individual decisions of millions and millions of ordinary people. People who were liberating themselves from constraints a decade ago are delaying gratification and imposing restraints on themselves and others.

Why? A better question is, why not? The historian Lawrence Stone, in "The Family, Sex, and Marriage in England 1500-1800," argues that people in England became more restrictive in the late 1500s, more permissive after the Restoration of 1660, and more restrictive again beginning around 1770. "Historical change is not a one-way street," he says and, writing in the middle 1970s, predicted "the cycle of history is revolving once more." As the excesses and costs of liberation become plain, people naturally apply restraint.

In the America of 1975-85, the excesses often appeared as threats to one's health. Scotch and steaks (red-meat consumption is way down too), cigarettes and marijuana all once seemed the emblems of the affluent, liberated life, of Hugh Hefner's Playboy pads. Now they seem dangerous. Americans have discovered, often tragically, what the history of venereal diseases should have told them: that whether it should or not, nature has a bias against promiscuous sex.

Lewis Thomas tells us how the development of penicillin in 1938 meant that physicians for the first time could cure many diseases thought incurable. And so Americans came to think that there was a pill that could cure any illness and a device -- a contraceptive, a shot, whatever -- that could guarantee pleasurable, risk-free sex. Now we are learning the limits our physical nature imposes on behavior and the penalties it exacts for excess.

We may be learning as well the limits our spiritual nature imposes, that it's not always best in the long run to maximize pleasure or freedom in the short run. As a society we're not eager to intrude in people's lives, but we're increasingly willing to penalize what we were reluctant a few years ago to regard as misconduct: witness recent laws to imprison drunk drivers, track down fathers delinquent in child-support payments, ban smoking in public places, ban or limit pornography.

Advocates of abortion, who once portrayed it as liberating and socially beneficial (see the first paragraphs of Justice Blackmun's opinion in Roe v. Wade, 1973), now argue for it as a lesser evil for people faced with anguishing dilemmas (the National Abortion Rights Action League's 1985 ad campaign).

Even so, Americans continue to talk as if we were still in an age of liberation. Advocates of liberation want to keep fighting and not acknowledge that their tide is ebbing; advocates of restraint have a stake in arguing that battles are still to be fought. Some surely are. But the evidence is accumulating. Whether opinion leaders admit it, we seem already to be in an Age of Restraint.