WOULD IT SURPRISE you to learn that more Americans, as a proportion of population, are in prison today than ever before? They are, and the increase in the prison population, as recorded by the Justice Department, is startling. From 1979 to 1982, the number of men and women in prison has increased from 314,000 to 394,000. That increase does not result simply from the increase in the population of young people, who are most likely to commit crimes. In 1970, 10 of every 10,000 Americans were in prison. In 1982, 17 of every 10,000 Americans were.

Since the government began keeping these statistics in 1926, the proportion of Americans in jails has reached peaks in recession years (1938-41, 1958-63) and has been lowest in wartime (1943-46, 1967-70). That makes a certain amount of sense: young males tend to be idle in recessions and tend to be occupied elsewhere in wartime. But even in recession, the proportion in prison never reached above 14 in 10,000. Obviously other things are at work.

Chief among them are changing attitudes toward crime and changing prison sentences. Prison admissions have risen far faster than prison releases because of mandatory sentencing laws, tougher sentencing under old laws and less generous parole practices. These changes have resulted, in large part, not only from the public's fear of crime, but also from its diminished sympathy for criminals. Fewer and fewer of us seem inclined to worry much about the unfortunate circumstances that may have led an individual to commit a crime, and fewer and fewer of us talk much about "the root causes of crime" any more. Instead, we are more likely to see crime as willful wrong acts that must be punished, and punished sternly. This change in attitudes, we think, has come not just from one segment of the population or from the holders of one set of political attitudes. It is a consensus, one that our sensitive political and judicial systems have translated into actions.

But such actions, while they may seem to solve one problem, can cause others. Prison overcrowding is greater than it used to be, and barbaric practices that no society should tolerate are common in prisons, as The Post's recent series on the Prince George's County jail reminded us. That means that state and local governments, sooner or later, will have to spend more money on prison facilities -- at just the same time that taxpayers are demanding limits on spending. Americans' increasingly stern attitude toward criminals may result in diverting public funds from enterprises such as education, which most voters look kindly on, to jails and prisons, which most voters begrudge money for. You can applaud the changing attitudes as reflected in the rise in prison population, but if you do, you should realize that it creates a whole new range of unpleasant problems.