Why did crime, which rose steadily thru the 1970s and 1980s, suddenly reverse itself and start dropping? Violent crime became a fact of life in American cities in the 1970s, and New York was especially scary. In 1979, New York City saw 16 murders committed in the subways alone.
And the problem seemed unsolvable. "There was not a lot of confidence there was going to be any major significant drop in crime," said Jim Finckenauer, professor of criminal justice at Rutgers University.
"The rise in crime, although something that troubled people, was pretty much taken as kind of the way it is," Finckenauer said.
But then, something surprising happened: throughout the 1990s, the crime rate plunged, and experts are still debating the reason for the drop in crime.
Steven Levitt and Stephen Dubner, the authors of "Freakonomics," think they know the real reason, and their conclusions have generated a fair amount of controversy. The drop in crime, they say, came about because of abortion.
How could abortion affect crime rates?
There was no shortage of politicians taking credit for making the streets safer for Americans.
President Clinton claimed the drop in crime was the result of an increased police presence in cities throughout the United States.
"He had called, remember, 100,000 police officers on the street, all sorts of crime initiatives -- he got to bathe in the glory of a reduction in crime," said Finckenauer.
And in California, Gov. Pete Wilson claimed the drop in crime was the result of the "three strikes and you're out" initiative that threw more repeat felony offenders in jail.
Back in New York, Mayor Rudolph Giuliani began getting tough on minor offenses. "It has to do with paying attention to the things we were ignoring in the past. Aggressive panhandling, squeegee operators, graffiti: You pay attention to these things, and all of a sudden you're gonna start to see murder come down by 30 [percent], 40 percent," Giuliani said.
But who was right?
"I spent about five years studying the different reasons why crime would fall. Was it police? Was it prisons? Was it the economy?" Levitt asked.
Levitt came to the conclusion that about 40 percent of crime's decline was the result of locking up a million more criminals. Fifteen percent of the drop was attributed to the waning of the crack epidemic. And roughly 10 percent could be credited to having more cops on the streets.
"What's left over -- 30 [percent] or 40 percent -- I actually believe is attributable to a cause that no one ever expected, which was the legalization of abortion," Levitt said.
"It's a very simple theory. Unwanted children are a tremendous risk for growing up and having criminal lives," said Levitt. "With the legalization of abortion, many fewer unwanted children were born, therefore, the children who were most at risk for being criminals -- they were never born."
The Supreme Court legalized abortion in 1973, but crime didn't begin to drop until some 18 years later. Levitt believes this can be attributed to the fact that young adults aged 16 to 24 commit the most crimes.
Levitt's theory has attracted its share of critics. Some considered it a racist statement.
"I was born to a poor single mother, and I became a presidential candidate and a minister. So why wouldn't he assume there'd be more of that?" asked the Rev. Al Sharpton.
Levitt was attacked by conservatives as well. "The response from the right was 'this is crazy because abortion is murder,'" Levitt said.
And even some economists find little to support in Levitt's theory. "Maybe abortion had some small effect. I don't think you can pull it out of the data," said Theodore J. Joyce, professor of economics and finance at Baruch College in New York.