Written by Noah Carl and Bo Winegard.
With fertility stuck below replacement level in most of the developed world, people are scrambling to understand which factors raise birth rates and which factors lower them. In a recent article, Daniel Hess, better known by his Twitter alias “More Births”, argues that high population density is a major cause of low birth rates. His argument stands in contrast to the one made by Bryan Caplan in Build, Baby, Build, namely that building at high density has several benefits and need not come at the expense of childbearing.
Hess presents an enormous amount of evidence in his article, which we will do our best to summarise here:
Various academic studies have examined the relationship between population density and fertility across countries or other geographic areas, and every one has found that it is negative. What’s more, some of these studies claim the relationship is causal.
Looking at fertility maps for Western countries like Britain, France, Germany, the US and Japan, the level is invariably lowest in highly dense urban centres. There is a strong negative association between fertility and urbanisation across countries. And there is a negative association between fertility and percentage living in apartments across major Western cities.
Low levels of fertility in highly dense urban centres cannot be explained entirely or even mostly by selection of families into the suburbs. Nor can they be explained by the cost of housing.
Studies of other mammal species have found that fertility declines when population density reaches a certain level. This suggests there may be general biological mechanisms that suppress reproduction at high densities.
Many economists who advocate building at high densities chose to raise their own families in single-family homes. And economics tells us to trust people’s revealed preferences over their stated preferences.
Hess’s article is comprehensive and well-argued. However, we are not convinced by his claims. While population density may have a small negative effect on fertility, we suspect that much of the effect claimed by Hess is in fact due to selection. We therefore doubt that population density is a major cause of low birth rates, although it may be a minor one.
