Fertility and the plough

"The current study finds that societies which historically engaged in plough agriculture today have lower fertility. We argue, and provide ethnographic evidence, that the finding is explained by the fact that with plough agriculture, children, like women, are relatively less useful in the field. The plough requires strength and eliminates the need for weeding, a task particularly suitable for women and children. This in turn generates a preference for fewer children, lowering fertility"--National Bureau of Economic Research web site.