I'm no expert but I would think that if the mechanism is abortion, it's such an important decision that everyone who wants to get one will do so. So then ~20 years later all the aborted criminals vanish from the statistics and stay gone, leaving the other criminals to draw their own stats.

I'm not saying abortion is the ONLY factor, just that if it is, you'd see a step (or sigmoid) in the crime data and then that's it. As in there's a constant density of abortion sensitive criminals until they get aborted.

Similarly if abortion became illegal again, you'd see a step the other way. I'm not familiar with it, but I've heard it's gotten very hard in some places in the US to get an abortion.