Can you cite a source for that? While there will be cases where that is true, it may also be that those without children have higher willpower (no oopses), for

Can you cite a source for that? While there will be cases where that is true, it may also be that those without children have higher willpower (no oopses), for example. Further, not all of the children's parents were younger than 30 when they had their child. Being childfree at 30 does not mean childfree at 40.