People are selective, especially when searching for long-term mates. Sometimes we respond unconsciously to fitness cues, as when we find symmetrical features and smooth skin attractive. But we also consciously select long-term partners based on shared values and interests. If people tend to marry those who share their basic values, and if the values that shape our political allegiance are heritable, then a small difference in fertility rates between different populations – that is, the number of children people choose to have – can grow into a large difference in the characteristics of a population over a few generations.

This is something that liberals in particular should worry about. There are fairly noticeable differences between the fertility rates in Red States vs Blue States in the U.S. (that is, in states with high proportions of liberals vs conservatives). Predominantly conservative states like Kansas, Oklahoma, and Alaska have a markedly higher number of births per capita than predominantly liberal states like Vermont and Massachusetts.