When it’s too crowded to have kids

Fushimi Inari Taisha, head shrine of Inari, the Japanese kami of fertility, among other things

Density can have profound effects on fertility. Population biologists call this phenomenon density dependence, and they’ve witnessed it in everything from single-celled organisms to elephants. It can influence fertility positively—individuals are more likely to meet mates in dense populations—or negatively—increased stress or lower food availability may drive fertility rates down. But despite evidence of the phenomenon in the natural world, little has been said about the its role in declining human fertility rates.

The relative paucity of studies examining density dependence in humans may be due in part to our persisting belief (conscious or unconscious) that in the course of developing culture, we have isolated ourselves from natural forces. Many previous studies of fertility rates tended to overlook what biologists see in the wild.

One study did, though. A demographic survey of around 150 countries, it uncovered strong evidence that population density is driving down human fertility rates. The authors accounted for all the usual variables found in fertility studies—infant mortality, gross domestic product per capita, percentage of women in the workforce, female literacy, and degree of urbanization. While those oft studied factors all still play a role, population density stood out as a new addition.

The researchers found that people throughout the world tend to have fewer kids when population densities are high, a pattern that repeated itself over the course of forty years. There were a few outliers—Australia has low population densities and low fertility while the Maldives has the opposite—but population density remained significant even when variables like infant mortality and GDP per capita included.

Density dependence was apparent even in the number of children people wanted, hinting that the cause may be more than just environmental. The authors used the Eurobarometer survey to see if people’s desires were aligned with population density. By and large, they were. People in sparsely populated Scandinavian countries desired more children, while people in the Netherlands wanted fewer. There were outliers, of course. The Irish continue to want larger families than average, the Germans fewer.

The exact mechanisms at work are still unknown. Density could be making food scarcer, or stress could be reducing fertility biochemically. Pollution may also be to blame. The psychological effects of crowding might be lowering libidos. Economics could be another driver. After all, many things are more expensive in higher density areas, whether that be food, shelter, child care, and so on. The truth is, we just don’t know at this point. But what should be clear is that culture and society have not insulated us from the forces of nature.


Lutz, W., Testa, M., & Penn, D. (2007). Population Density is a Key Factor in Declining Human Fertility Population and Environment, 28 (2), 69-81 DOI: 10.1007/s11111-007-0037-6

Photo by Miguel Michán.

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