When it came out in 2005, one of the most contentious topics presented in the book Freakonomics was Steven Levitt & John Donohue's hypothesis on the link between unwanted pregnancies, legalized abortion, and crime. For the uninitiated, Levitt & Donohue published their paper, "The Impact of Legalized Abortion on Crime" in 2001, stating that "Legalized abortion appears to account for as much as 50 percent of the recent drop in crime."
For many, the premise of legalized-abortions-equals-less crime elicits a knee-jerk reaction that may prevent some from reading further, the underlying idea is that unwanted pregnancies could lead to unwanted kids that could eventually go on to become criminals. According to Freakonomics co-author Steven Dubner:
"What did Donohue mean by "unwantedness"? He was referring to the expansive social-sciences literature which showed that children born to parents who didn't truly want that child, or weren't ready for that child, these children were more likely to have worse outcomes as they grew up — health and education outcomes. But also, these so-called "unwanted" kids would ultimately be more likely to engage in criminal behaviors."
I do have to note that Levitt & Donohue's original paper published in 2001 came under scrutiny by their peers for quite a few reasons, most notably a coding error was discovered that eventually overshadowed the whole premise of their paper. The error turned out to be a transposed column in a table that was eventually corrected; however, the damage had been done and many dismissed the whole paper until the topic resurfaced again in 2005 with the publication of Freakonomics. Nearly two decades later, Levitt & Donohue revisited this topic in their 2019 paper titled "The Impact of Legalized Abortion on Crime Over the Last Two Decades", where the original approach applied to the 1982-1997 data set was applied to data gathered from 1997-2014, which showed an even stronger result with a reduction in violent crime by 47% and a reduction in property crime by 33%.
With SCOTUS overturning the Roe v. Wade decision, we will now get a chance in 15 to 20 years to see how accurate Levit & Donohue's hypothesis was after comparing data from the states that banned abortion versus the rest of the United States.