Does racial diversity cause lower fertility?

Finding a spouse is harder when you are around genetic strangers

I know, it sounds like a far right conspiracy theory, however:

• Gurun, U. G., & Solomon, D. H. (2024). E Pluribus, Pauciores (Out of Many, Fewer): Diversity and Birth Rates. Pauciores (Out of Many, Fewer): Diversity and Birth Rates (July 01, 2024).

In the United States, local measures of racial and ethnic diversity are robustly associated with lower birth rates. A one standard deviation decrease in racial concentration (having people of many different races nearby) or increase in racial isolation (being from a numerically smaller race in that area) is associated with 0.064 and 0.044 fewer children, respectively, after controlling for many other drivers of birth rates. Racial isolation effects hold within an area and year, suggesting that they are not just proxies for omitted local characteristics. This pattern holds across racial groups, is present in different vintages of the US census data (including before the Civil War), and holds internationally. Diversity is associated with lower marriage rates and marrying later. These patterns are related to homophily (the tendency to marry people of the same race), as the effects are stronger in races that intermarry less and vary with sex differences in intermarriage. The rise in racial diversity in the US since 1970 explains 44% of the decline in birth rates during that period, and 89% of the drop since 2006.

They go through a number of datasets from different countries and time periods with relatively rigorous economic methods and the results are fairly consistent in showing that racial diversity, or rather, the relative size (proportion) of an ethnic group in some place at some time can predict its fertility rate. There are lots of tables like this one:

All the p values are small enough to be convincing this isn't a fishing expedition study. One might be skeptical that the effect sizes differ between models, which makes it hard to accept they are estimating the size of the same causal effect. However, I think this is because of the changing size of the spatial units (area vs. county vs. state) and period analyzed (1850 vs. 1980).

Why this pattern? As technology improved, it became possible to marry people who lived further away. Historically, one could only really marry someone in one's small neighboring area, but then we get horse carriages, steam powered trains, personal cars, diesel trains, and eventually we ended up with cheap flights, enabling even people from different continents to date. Dutch historical data show this increasing distance across time:

It's amazing to think the birthplace distance between newlyweds used to be only about 4 km on average, less than an hour's walk. The Dutch study ends with 1920 data, but a modern study from Sweden found that "it is still the case that half of all partners lived within 9 kilometres of each other before moving in together".

The next thing is the very strong assortative mating for ethnicity or race, here illustrated with American data:

Putting these facts together, you will get the implication that belonging to a small ethnic group in some location will hinder your dating market given people's selection for partners who are close by and of the same ethnic group. Hypothetically then, if we moved all Americans (say) into ethnically homogeneous blocks (strict segregation), fertility rates would slightly increase. To be fair, the world and countries already have quite strong ethnic homogeneity in terms of where they live. Take New York, a cosmopolitan city and look up where people live according to their race on the Racial Dot Map:

Blue clusters are Asians, yellow are Blacks, brown are Whites, Turquoise are Hispanics. This is not an American phenomenon due to legacy of Jim Crow or slavery or anti-Asian immigration laws, here's South Africa's Cape Town:

Green clusters are Blacks, Purple are Whites, Yellow are Coloreds.

On top of the inability to find a matching spouse from the same ethnic group, we also know that interracial marriages are less stable (from my prior post):

To this can be added that interracial marriages also produce fewer children (American data):

I cropped the full table because it is too long, but model 3 adds controls for age, immigration status, education, cohabitation history, and year, and model 4 adds controls for neighborhood racial proportions. Probably models 3-4 produce results closer to what we care about (though controlling for education is bad).

To read this, compare the values for the mixed race lines vs. the same race equivalent. For instance, from model 3, Asians together 0.98, vs. Asian + 0.81/0.89.

Furthermore, more genetically distant marriages produce fewer children even within ethnic groups, here Icelandic data:

We can also note that people living in cities, which typically has much higher racial diversity, have lower fertility. A bit paradoxical since they are surrounded by people, but many of them of the wrong race for optimal pairing, statistically speaking.

Overall, taking the new findings about racial diversity in one's location and fertility together with the plausible mechanism (and the authors provide evidence of this), it seems likely that racial diversity has a negative causal effect on marriages and fertility rates. From a pro-natalist perspective then, it seems that racial segregation is a good thing.

Of course, it can be mentioned that political ideology, religiousness, and general traditional mindedness influence both variables, as the more conservative people will marry within group more and also have more children, even if they don't marry within group. This would create a spurious association between racial diversity (lower in conservative areas) and fertility (higher in conservative areas). In the new study, one could control for many things that reduce this confounding bias, but we can't be entirely certain how much it affects the results.