You may be thinking: Yes, living under crowded conditions surely drives people crazy. And the reason may be traced back to some unfortunate rats.

In the mid-20th century ethologist John Calhoun wanted to see how overcrowding would influence social behavior in rats. He placed rats in a confined space and allowed them to multiply with relatively little control. The results looked like scenes out of a horror movie: cannibalism, dead infants and complete social withdrawal, to name a few.

Calhoun’s rats captured public imagination and inspired a surge of research on the psychological effects of density in our own species. Some studies found that people living in crowded environments indeed showed a variety of social pathologies, just like Calhoun’s rats. But other studies did not. Reviews of the early research concluded that popular fears about overcrowding may be unfounded.

Now half a century has passed, and the world population has doubled. On the other hand, research on the psychological effects of density has all but disappeared.

We revisited this old topic in a recent paper, this time with a tool called life history theory. It is about how all animals allocate their limited time and energy across life’s tasks, such as growing, mating and parenting. Aspects of the environment shape these allocation choices.

What does this have to do with density? One of life history theory’s earliest ideas was that environments of low density—where there are few individuals around—would favor organisms that adopt a “fast” life history strategy. This strategy focuses on quick reproduction and having many offspring but with little investment in each. Put simply, it is focused on the present and prioritizes “quantity over quality.”

A low-density environment favors a fast strategy because it is presumed to have abundant resources with little social competition. Here fast reproduction would allow for full exploitation of the environment’s resources. Animals living in low-density environments also would not need to invest much in offspring, because it would be easy for those offspring to survive independently in such an environment.

But things get different when the environment gets crowded and strong social competition for resources and territory exists. To successfully compete, individuals now need to spend more time and energy building their own abilities. This often leads to a delay in reproduction. In a dense environment, one’s offspring also face greater social competition. Hence, it may be more adaptive to focus time and energy on just a few offspring (to increase their abilities and competitiveness) instead of spreading resources over many offspring.

This approach is referred to as a “slow” life history strategy, and it prioritizes “quality over quantity.” A slow life history strategy also involves a psychology that plans for the future, given the need to build one’s abilities over time. Our question was therefore a simple one: Would higher densities also lead people to adopt a slower life history?

We examined this idea in a variety of ways. First, we gathered data on country-level population densities and on a variety of psychological traits and behaviors related to life history. We did the same thing for the 50 U.S. states, where equivalent data were available.

Indeed, we found that across countries and across U.S. states, individuals in regions with denser populations showed traits that corresponded to the psychological profile of a slower life history. They were more likely to plan for the future, preferred long-term, committed romantic relationships, married later, had fewer children, and were more likely to invest in both their own and their children’s education. These relationships held when taking into account alternative factors, such as economic development and urbanization.

To see if there might be similar effects in short-term situations, we conducted experiments in which we had both undergraduates and slightly older adults read an article that talked about increasing population growth in the U.S. After reading the article, participants reported both their romantic relationship and family-size preferences. We found that the undergraduates who read the density article preferred having a few committed romantic relationships (instead of many casual ones). The older adults who read the same article preferred to have fewer children and to invest more in each child (instead of investing less in many children).

Thus, in experiments, individuals led to think about increasing population densities also seemed to shift toward a slower life history, characterized by quality over quantity. 

Many of us have intuitions about the effects of crowdedness. It is therefore useful to anticipate some questions. For instance, will higher densities always lead to a slow life history? No. In fact, when high densities are paired with unpredictable death or disease, life history theory predicts that a faster life history will emerge. A second critical point to consider is the nature of social competition. The assumption is that humans typically compete for resources by building skills and abilities (for example, through education). But this might not always be the case. In environments where competition is carried out by forms of lethal violence, we would once again expect higher densities to lead to a faster life history.

These are just some of the many unanswered questions about density. More important, our current work presents a new way of thinking about and understanding the psychological effects of population density. In addition, it elucidates how population density might underlie psychological differences across societies and human groups in general. The hope is that our research will generate renewed interest in the study of density’s psychological impact.

That said, perhaps a crowded life does drive people a little crazy—but not in the dystopian ways we expected from Calhoun’s rats. Instead it may make people obsessed about planning for the future, getting a good education, waiting for that perfect romantic partner and putting everything they have into that one child who is going to make them proud.