As soon as I read the headline (having read Freaknomics myself) I knew exactly what the author would be getting at. And indeed, so it was: just as the introduction of abortion led to a decrease in crime after a suitable time-lag, so equally the abolition of abortion will likely result in an increase of crime.
I thought one reason for the reduction in crime was the introduction of unleaded fuel in the 90s. The lead tetroxide anti-knocking agent in leaded fuel prior to this had pulled down IQ.