WHAT PART of America has had the biggest population explosion in the last dozen years? It may be the prisons. At the end of 1984, there were 463,000 Americans in prison, more than twice as many as the 196,000 of 12 years before. This increase is all the more striking because the population that commits most crimes and is most likely to be incarcerated -- men from their mid-teens to late 20s -- increased by only a little more than 10 percent over that time.

The causes and effects of this massive and, in the last 50 years, unprecedented change tell us interesting things about our society. For the number of people in prison is not the result of a single decision. It is the collective result of hundreds of thousands of decisions, made by local juries, state legislators, voters, judges and prosecutors and police officers. A national administration can make some difference. But only 4 percent of prisoners are in federal institutions. The mandatory-sentence laws and longer terms that are the major factor in swelling the prison population are local initiatives, supported by politicians from the far right all the way to President Reagan's successor in California, Jerry Brown. Decisions to build new prisons are made mostly by state officials. Changes in prison populations are the sum total of millions of changes of mind.

To some extent, larger prison populations reflect an increase in crime rates over the years and greater success in apprehending suspects and trying defendants. But note that the rise in crime rates in the middle 1960s was accompanied by a drop, not a rise in the prison population, from 210,000 in 1965 to below 200,000 through 1972. Those were the years when the crime issue was on almost every politician's lips. Yet the criminal justice system, representing a wide range of views from all parts of society, was imprisoning fewer people. Note also that the rise in prison population didn't start with the inauguration of the Reagan administration. The figure rose to 250,000 in 1976, passed the 300,000 mark in 1979, hit 350,000 in 1981 and 400,000 in 1983, and surged past 450,000 in 1984.

One clear effect of this change has been the falling crime rate of the 1980s -- falling much faster than the number of men in the high-crime age groups. Some of the people now in jail would surely be committing crimes if they were out. Other effects may not be as benign, but they're also worth thinking about. One is the likelihood that more people are being held unjustly. Another is that the mandatory-sentence laws and tougher sentencing by some judges are imposing unjustly long and disparate sentences on offenders. A third is that conditions inside what often must be overcrowded prisons may be inhumane.

None of these problems may seem the first order of business to citizens who are cheered by the decrease in crime. But they are necessary orders of business in a society that cherishes its reputation for fairness and humanity.