Good point, but technically you really want to compare like with like. Maybe prisoners would have had even more kids than they did had they not been imprisoned but allowed to lead their life of crime unpunished. In fact that seems probable. Imagine this: you are told that you belong to the control arm of such a study, which means that if you commit any crimes or other infractions, you will never be punished. What would you do and what would be the result? I, being at heart bad, very bad, would get very rich very quickly (not telling you how, but it involves being above the law), and I would have plenty of unprotected sex (with carefully selected gold diggers) without worrying about child care payments.