This is likely a myth; crime dropped all across the developed world in a similar timeframe, but dates of legalisation of abortion likely don't line up. One popular speculation is the phasing out of leaded petrol, but really this one seems to remain a case of "shrug, dunno".
The world is complex and most phenomenons are high dimensional.
It’s very likely:
- criminal potential populations were reduced
- economics lead to stable options for more individuals in the late 90s
- lead was removed
- a myriad of other improvements in society that generally led to
Less crime
