It also doesn't have to be at the level of genes. Countries and religions apply varying levels of restriction on birth control, and make deciding to be a paren

It also doesn't have to be at the level of genes. Countries and religions apply varying levels of restriction on birth control, and make deciding to be a parent more or less desirable. All else equal, the groups that encourage having kids and discourage birth control will grow, and the others will shrink on a relative basis.