James Taranto has posited an interesting theory on abortion. The gist is that more liberals have abortions than conservatives. Children form their views due to their parents. It is most likely that a liberal parent will have a liberal child and a conservative parent will have a conservative child. Therefore, the liberal birth rate is slower due to the rate of abortion.
A number of statisticians have noted that the Democratic voting population grew continuously from the 1940s until the 1970s, the period of Roe. In the 1970s, through the present, the votes that Democrats have received has declined.
Donald, over at Back of the Envelope, has tried to quantity the Roe effect. The data are mildly persuasive. While only mildly persuasive on its own, it adds to data that James Taranto has put forward. They seem to be on to something about which no liberal would ever admit.