High population density might induce better habits, according to some new research at the University of Michigan. If so, that's good news for the residents of an ever more highly populated world—and a big surprise for a generation of social critics.

"Popollution" threatens to destroy the planet, Larry Gordon warned in his 1982 presidential address to the American Public Health Association. "When we consider the problems of hunger, poverty, depletion of resources, and overcrowding among the residents of our planet today, the future of human welfare looks grim indeed," he declared.

Overcrowding was a big concern for those 20th-century prophets of population doom. In 1962, National Institute of Mental Health researcher John Calhoun published an influential article, "Population Density and Social Pathology," in Scientific American. Calhoun had conducted experiments in which he monitored overcrowded rats. As population density increased, female rats became less able to carry pregnancies to full term—and they so neglected the pups that were born that most died. Calhoun also documented increasing behavioral disturbances among the male rats, ranging "from sexual deviation to cannibalism and from frenetic overactivity to a pathological withdrawal." All of these pathologies amounted to a "behavioral sink" in which infant mortality ran as high as 96 percent.

Calhoun's work was cited both by professional researchers and by overpopulation popularizers. Gordon, for example, argued that "too many members of the human species are already being destroyed by violence in overpopulated areas in the same manner as suggested by laboratory research utilizing other animals." In his 1961 book The City in History, the anti-modernist critic Lewis Mumford cited "scientific experiments in rats—for when they are placed in equally congested quarters, they exhibit the same symptoms of stress, alienation, hostility, sexual perversion, parental incompetence, and rabid violence that we now find in the Megapolis."

In The Pump House Gang (1968), the hipster journalist Tom Wolfe referenced Calhoun's behavioral sink: "Overcrowding gets the adrenalin going, and the adrenalin gets them hyped up. And here they are, hyped up, turning bilious, nephritic, queer, autistic, sadistic, barren, batty, sloppy, hot-in-the-pants, chancred-on-the-flankers, leering, puling, numb…" And in his 1968 screed The Population Bomb, the Stanford biologist Paul Ehrlich declared that he had come to emotionally understand the population explosion "one stinking hot night in Delhi" during a taxi ride to his hotel. "The streets seemed alive with people. People eating, people washing, people sleeping. People visiting, arguing, and screaming. People thrusting their hands through the taxi window, begging. People defecating and urinating. People clinging to buses. People herding animals. People, people, people, people….[T]he dust, the noise, heat, and cooking fires gave the scene a hellish prospect."

The theme of dystopian overcrowding inspired many popular books in the 1960s and 1970s, note the London School of Economics historians Edmund Ramsden and Jon Adams. Among the texts they cite are Terracide (1970), by Ron M. Linton; My Petition for More Space (1974), by John Hersey; and the novels Make Room! Make Room! (1966), by Harry Harrison; Logan's Run (1967), by William Nolan and George Johnson; and Stand on Zanzibar (1968), by John Brunner.

But now, in stark contrast to these visions of chaos and collapse, new research suggests that increased population density isn't a disaster at all. Indeed, it's channeling human efforts and aspirations in productive directions. So says a report by a team of researchers led by Oliver Sng, a social psychologist at the University of Michigan.

In "The Crowded Life Is a Slow Life," a new paper published in the Journal of Personality and Social Psychology, Sng and his colleagues probe how life history strategies change as population densities increase. "Life history" involves the tradeoffs individual organisms must make as they allocate effort and time to such tasks as growth, bodily maintenance, and reproduction. Some species, such as rats, are subject to high levels of predation and live only a short while; given their ecological niche, rats have evolved to pursue a fast life history strategy, in which they mature rapidly, bear lots of offspring, and invest little in each one. Elephants, on the other hand, are less subject to predators and live longer, so they follow a slow life history strategy in which they mature over a longer time and have fewer offspring, in which they invest more time and care.

Sng and his colleagues asked, Does human life history strategy shift when population density increases? For most species, low density generally leads to faster reproduction and higher density slows reproduction, because more parental investment is needed to boost the chances that offspring will become competitive enough to survive and reproduce. Their results with regard to how people behave are very preliminary, but nonetheless intriguing.

First, the researchers probed how future orientation, investment in education, long-term mating orientation, marriage age, fertility, and parental investment shift as density increases among people. They compared these variables across countries and then across U.S. states with differing population densities. They devised various life history indicators based on data from such sources as the sociosexual orientation inventory from 48 countries, life expectancy, adolescent birth rates, preprimary school enrollment, future orientation, and so forth. The researchers then compared the indicators' prevalence and strength across nations to see how they vary with population density. To focus on the effects of population density, they use various statistical techniques to remove the confounding effects of wealth and urbanization.

They found that people living in countries and states with denser populations are more likely, on average, to pursue a slow life history strategy. That is, they have fewer sexual partners, get married later, postpone children, have fewer children, invest more in education for both themselves and their kids, and save more for retirement.

Next Sng and his colleagues conducted several experiments to compare the future orientation of subjects who feel crowded to subjects who had neutral experiences. In one experiment, a set of students was assigned to read a rather claustrophobic article titled "The Crowded Life: Too Many, Too Much" and the comparison group did not. To gage their future time orientation, both groups where asked questions like, "Would you want to get $100 tomorrow or $150 90 days from now?" The group that read the article was more likely to say that they were more inclined to wait for the larger reward.

In another experiment, the researchers played sound clips lasting one minute; one with loud crowd conversational hubbub and another with white noise. In this case, those hearing the crowd noises marginally opted to wait for a larger cash reward.

In the last two experiments, the researchers conducted experiments to see if inducing feelings of crowdedness would result in shifts among experimental subjects on mating, parenting, and educational strategies. Since individuals would be at various stages of their life histories, the experimenters sorted the subjects by age. In one experiment, they had subjects read two different versions of "The Crowded Life" essay; one discussing overpopulation with respect to people and other with squirrels. Younger subjects who read about growing crowds of people tended to choose subsequent survey items suggesting that they would prefer to have fewer sexual partners and spend more time with them. Older subjects cued with increasing density expressed a preference for fewer children and more schooling for them.

These very preliminary results need to be taken with good-sized helping of salt, but they are suggestive. As the researchers conclude, "Our findings may hold important societal implications. As populations worldwide continue to grow, density continues to increase. One might thus predict, all else equal, a global trend toward slower life history strategies."

At the time Calhoun was running his rat experiments, world population stood at just over 3 billion people. Today there are 7.4 billion of us living on Earth. As of yet, our "overcrowded" world has not spun out of control. In fact, quite the opposite: Absolute poverty globally has fallen below 10 percent for the first time in history and large-scale violence is arguably hovering around its lowest level ever. For people, unlike rats, the human "behavioral sink" seems to be greater creativity, not pathological collapse.