On Jan. 1, 1900, there were 57,070 people locked up in local, state and federal jails and prisons in the United States. That was 122 inmates for every 100,000 Americans.

As of this Jan. 1, a new study estimates, there are 1,982,084 adults in U.S. jails and prisons. That's 725 inmates for every 100,000 Americans.

Before 2000 is two months old, America's imprisoned population will reach 2 million--probably hitting that level on Feb. 15, the study predicts. By the end of 2000, if rates continue, the nation's inmate population will reach 2,073,969.

"Our incarceration binge is America's real Y2K problem," said Jason Ziedenberg, co-author of the study published this month by the Washington-based Justice Policy Institute. "As we approach 2 million prisoners in 2000, we have to find alternatives to incarceration to solve America's social problems."

The cost of housing inmates will soon exceed $40 billion a year, the study found, and state governments invariably are spending more on prisons and jails than on colleges and universities.

"As we enter the new millennium, the ascendance of prisons as our decade's major public works project and social program is a sad legacy," said Vincent Schiraldi, director of the institute, in the report titled "The Punishing Decade: Prison and Jail Estimates at the Millennium."

The institute describes itself as "a policy development and research body, which promotes effective and sensible approaches to America's justice system." Others in the criminal justice field generally view it as a liberal think tank supported largely by liberal foundations.

Political ideology aside, the institute's research is based largely on nonpolitical statistics from government records depicting the results of new laws, including mandatory minimum prison sentences for a range of crimes--especially those involving drugs and guns. By prescribing the fixed minimum term to be imposed upon conviction of a specific crime--such as 15 years in prison for selling two ounces of cocaine--the laws prohibit judges from considering extenuating circumstances and have imprisoned an inordinate number of first-time, nonviolent offenders.

Researchers say any correlation between incarceration and crime rates remains elusive. The institute's recent study contrasted New York and California, for instance.

Between 1992 and 1997, New York state's violent crime rate fell by 38.6 percent and its murder rate by 54.5 percent. During this period, New York's prison population was growing by 30 inmates a week. At the same time, California's prison population was growing by 270 inmates per week while the state's violent crime rate was falling 23 percent and its murder rate by 28 percent.