The Abortion hypothesis: kids who would've otherwise turned out to be violent criminals were aborted:
https://en.wikipedia.org/wiki/Legalized_abortion_and_crime_e...
The Broken Windows hypothesis, that cracking down on minor petty crime reduces major violent crime:
https://en.wikipedia.org/wiki/Broken_windows_theory
The Economic Development hypothesis, that tough times create crime:
https://www.citylab.com/life/2013/09/puzzling-relationship-b...
And probably a bunch that I haven't heard of.
Really, I doubt we'll ever figure out what causes an increase or decrease in the crime rate, because there's likely more than one cause. Because of that, I don't think it's good evidence on either side for a debate on spanking. (Which, BTW, has just validated the point everyone's been making that any collaboratively-built parenting encyclopedia is likely to generate more heat than light.)
