SuperFreakonomics attributed the dramatic drop in crime in the 1990s to Roe vs Wade. http://www.freakonomics.com/2005/05/15/abortion-and-crime-wh...
http://en.wikipedia.org/wiki/The_Impact_of_Legalized_Abortio... A number of people disagree with that conclusion based on seemingly reasonable scientific arguments.
If you start with the understanding that a baby in the womb is a human, this arguments sounds like "it's good that we have legal murder because it prevents illegal murder".