Science has long presumed that children living in cities grow faster and healthier than rural kids—but that trend has flipped over the past three decades, a new study suggests. A global study published Wednesday in Nature found that the average height of urban children and adolescents ages 5 to 19 is now slightly shorter than that of their peers in rural areas in most countries—notably in wealthy countries such as the U.S., the U.K. and France.

“Where we've historically seen a quite clear benefit for living in cities, that benefit has been slightly diminished over time,” says study co-author Honor Bixby, a population health and epidemiology research fellow at the University of Essex in England. “But it can be viewed as a positive in that rural height is really catching up.”

Researchers are still trying to tease apart exactly why this is happening, however.

Cities have long been associated with better health—researchers call this the “urban advantage.” Residents of bustling, developed centers would seem likely to have better access to quality health care, education, safety and nutrition, and barriers to these resources can especially affect children's crucial early growth. “This early stage of life really sets the tone for health in adulthood and later life,” says Bixby, who worked on the study along with more than 1,500 researchers in the Noncommunicable Disease Risk Factor Collaboration, a worldwide network of scientists and physicians. “We specifically look at height and body mass index (BMI) as anthropometric measures of growth and development because [height and weight are] both influenced by the quality of nutrition but also the healthiness of the living environment.” (Some experts and members of the public have criticized the limitations of BMI, its overuse as an accurate assessment of health and its inability to capture variability between individuals. Bixby says it can still be a helpful, however, in estimating averages and trends on a population level.)

The study analyzed the data of 71 million participants from 2,325 population studies conducted between 1990 and 2020 across 200 countries and territories. In 1990, kids living in cities were taller than those in rural areas—though in most high-income countries, the difference was negligible. By 2020, however, while heights generally increased in children in both urban and rural areas, children in cities saw lower height gains in most countries.

“What has come out as the shining result, especially since the differences in BMI actually turned out to be far less, was height,” Bixby says. “When we're talking about a small urban disadvantage, we're saying that the average height in the population living in cities is slightly less than the average height of someone living in a rural area in the same country.”

The change could signify that the gap in health resources between the two populations has been closing. But is it because the health of people in cities is declining or because the health of those in rural areas is catching up? It's also difficult to determine if the changes were caused by socioeconomic factors or population shifts—or a mixture of both, says Mahesh Karra, an assistant professor of global development policy at Boston University, who was not involved in the new study.

Natural migration and expansion of urban development are factors that could influence who lives in cities today. According to a 2018 United Nations report, 55 percent of the world's population then lived in urban areas. “For the first time in human history, the majority of people live in urban areas, but that now changes the relative composition of who is living in a rural area versus an urban area,” Karra says. “Compositions are also changing because there's a lot of rural-to-urban migration. People are much more mobile these days, and it becomes [difficult to] disentangle these average effects.”

Bixby says migration could underlie some of the changes but is probably not a main factor in the recent trends. Past migration studies, she says, have shown that over time, newcomers eventually take on the same health characteristics of the population they move to because they gain access to the same services. For example, kids who move to a city will go to the same schools and be closer to the same services as those who were already living there. “There is more that could be said about where migrants fall on the socioeconomic distribution, and whether they can fully access the benefits of cities, but then that is more about socioeconomic disparities than migration per se,” Bixby says. “It is true though that we just don't have the data to be able to properly quantify the role of migration in our study.”

Karra agrees that migration is probably not the main driver of the height change in cities. “The cynical view is if you imagine sicker people are moving from rural to urban areas because they know that urban areas [provide] better care,” he explains. This could, in theory, bring down the health level in cities and narrow the gap between populations. “But that is if urban areas are falling in terms of their progression,” Karra adds. “For the most part, you see that in both urban and rural areas, health is improving; BMI and height is increasing over time. But it's increasing faster in rural areas than it is in urban areas, and that's the convergence where you're now starting to see either better health or better selection of the composition of people who continue to remain in rural areas.”

Bixby adds that countries where rural height improvements were most apparent might be experiencing economic growth or investment in raising living standards. Although the overall height increase in both rural and urban areas is a positive trend, she emphasizes that disparities continue to persist—particularly in countries in sub-Saharan Africa, South Asia, the Pacific and the Middle East. Boys living in rural places in these regions did not grow taller, and in some countries in these areas, the average height of boys became shorter regardless of where they lived.

“It's not that we've closed gaps, it's just that we see convergence. And that, I think, is a pretty important [distinction] to say,” Karra says. In low-income countries, “rural children still may be marginally lagging behind, and so there are still opportunities for continuing to think about vulnerable population targeting, where resources will be used more effectively. That also comes back to us having to dig a little deeper about exactly who are the kids who are lagging behind.”

Bixby and her colleagues are also working to better understand and focus on where the poorest populations live and how to support them. She hopes the data will provide guidance on where to carry out policies and programs that support growth and development.

“Even in wealthy nations, the inequality that we see within cities can be really huge,” she says. “This is no real surprise, but while it's the case that cities can offer many opportunities for good health, these are opportunities that seem to not be accessible to everyone. Those missing out on those opportunities are often the most deprived people. I think that is what is coming out in the data, and it is a kind of a warning sign for growing inequalities within cities.”

Editor's Note (3/29/23): This story was edited after posting to correct the number of decades covered by the study.