The case for: If you’ve heard of Freakonomics, you know this one. In 1994, when the crime rate started to drop, a child born in 1973 was 21 — and a child who wasn’t born in 1973, because the mother got a newly legal abortion after Roe v. Wade, was one fewer 21-year-old in the population. It’s a very appealing argument to people who hate obvious answers. It’s also, interestingly, one of the few theories on this list with strong empirical support from other countries: Studies as far away as Romania have found the same effect.
The case against: There are two big problems with the abortion theory. One of them is that abortions didn’t suddenly go from 0 to 60 when abortion was legalized. Before Roe v. Wade, illegal abortions still happened; after the decision, there were still plenty of people who chose not to get them. There was also a supply problem: “It wasn’t like Roe v. Wade was decided and suddenly there were a million places to get an abortion,” John Roman of the Urban Institute says.
The second problem is age. In the 1990s, the crime rate didn’t just go down among people born in 1973 or later. It went down among a bunch of age groups at once. There’s evidence that youth crime rates influence the crime rates of older adults, but that doesn’t mean the people who turned 21 in 1994 were powerful enough to suppress crime among people a decade older.
This theory also makes assumptions about abortion that aren’t necessarily true. The unspoken assumption of the abortion theory is that abortions necessarily prevent unwanted children, and unwanted children necessarily commit more crime. Those aren’t proven assumptions.
The bottom line: Probably some effect in the 1990s. Like many of these theories, there’s empirical support that abortion reduced crime to some extent in the 1990s. (Because there isn’t detailed-enough data on abortions, it’s hard to quantify exactly how much.) But crime continued to decline in the 2000s, after the Roe v. Wade generation was out of prime criminal age — making it unlikely that abortion explains why crime continued to drop through the first 15 years of the 21st century.