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America’s big decline in crime, explained

Crime has plummeted in America over the past couple decades. Here are potential explanations for why it happened.

Since the 1990s, America has undergone an incredible transformation: Violent crime and murders have dropped by roughly half across the country.

Why? We actually don’t know for sure. But criminologists have put forward some theories.

Some, like mass incarceration, seemed pretty solid in the 1990s, but have been called into question as more data has come in. Others, like the idea that more guns mean less crime, never had much, if any, good research behind them. Meanwhile, some new theories — like lead getting taken out of gasoline — have gotten very popular.

Chances are that no one change is to credit for the whole crime drop. Instead, a lot of factors likely worked together to produce America’s steep decline in crime.

Here is a look at some of the major factors that have been credited over the past few years, as well as the evidence behind them.

Dara Lind and German Lopez