America put more people in prison in the 1990s than in any decade in its history. That started a debate over the wisdom of spending billons of dollars to keep nearly 2 million people locked up. According to statistics, the United States ends 1999 with 1, 983, 084 men and women in prisons. That shows an increase nearly 840, 000 prisoners during the 1990s and makes the United States the country with the highest prison population in the world. With the cost of housing a prisoner at about MYM20, 000 a year, the cost in 1999 keeping all these prisoners behind bar is about MYM39 billion. Some experts argue that the money is well spent, saying the cost of keeping prisoners behind back doesn’t seem much in comparison to the cost of crime to society. They also point out that the rise in the prisoner population in the 1990s coincided with(与……相一致)a steady drop in the US crime rates. It is reported that serious crime has decreased for seven years continuously. "There are a noticeable number of people who don’t do crimes because they don’t want to go to prison, " they say. There is a heated debate among American experts because______.