Actually Michael (P), this won't work for their argument; the drop in the overall birth rate means nothing if the percentage of demon spawn most prone to crime (due to bad mothering? genes?) nonetheless rises. Or to put it another way: Even if the percentage of 18-25 year olds were increasing, they would predict a reduction in crime as long as the crime prone ones were being nixed in the womb.
That is, they attribute to abortion primary causality because it is used in such a race and class specific way that it eliminates the unwanted demons whom ill educated, often minority and poor women have conceived on account of their ignorance (it is implied) of birth control or irresponsibility of their wild animal partners. Abortion is treated here as a prophylactic against dysgenics--a weapon against differential fertility that could leave you with rising numbers of demon spawn even as the absolute number of 18-25 years old falls and thus a 10-20% higher crime rate (as they counterfactually estimate). This is essentially the Charles Murray program in which every attempt at birth control by the underclass is supported. There is no reason to expect that these authors could not and will not argue that family caps too will make an important contribution to crime reduction. But best for this to be implied due to the terrible forces of political correctness.
But the Charles Murray world view doesn't stop there. In terms of its immigration position--pushing the low skilled out on the basis of insufficient immigration points--it has found an advocate in Harvard's George (Jorge) Borjas whose new book Heaven's Gate is just out. Peter Brimelow seems to have a respectable academic to join him. Borjas goes so far to suggest to defend tight borders on the grounds that competition from immigrants at the bottom end of the labor redistributes income up (perhaps that's why WJ Wilson is willing to take the book seriously), but then why not fight for a higher minimum wage? Do you think he is being cynical?
Yours, Rakesh