Freakonomics argued that crime correlates to whether or not abortion is available.
If it is not, crime rates are up, and by a lot.
If it is, crime rates are down.
When you flip from one to the other, takes about 15/20 years for the effect to show up.
Rationale is that forcing parents to have their kids when they're not ready for them significantly increases delinquency in young adults.
This is apparently the only possible theory at the moment. It's not proven, of course, but the other theories which were given have been found lacking. This is the only theory which has some evidence, and hasn't been found to be wrong.
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