As a child develops, a tug of war between genes and environment settles the issue of the child's intelligence. One theory on how that struggle plays out proposes that among advantaged kids—with the pull of educational resources—DNA largely wins, allowing genetic variation to settle smarts. At the other end of the economic spectrum, the strong arm of poverty drags down genetic potential in the disadvantaged.

But over the years, researchers have gone back and forth on this theory, called the Scarr-Rowe hypothesis. It has held up in some studies, but inexplicably slipped away in others, leaving researchers puzzled over the deciding factors in the nature-vs-nurture battle. Now, researchers think they know why.

In a new meta-analysis of 14 psychology studies from the past few decades, researchers found that the strength of poverty’s pull differed by country, with US poverty providing the only forceful yank among developed nations. The authors, who published the results in Psychological Science, speculate that the wider inequalities in education and medical access in the US may explain poverty’s extra power. The finding could not only resolve the data discrepancies of the past, but it may also lead researchers to a more nuanced understanding of poverty’s effects on IQ and how to thwart them.

“It’s a terrific meta-analysis,” psychologist Eric Turkheimer of the University of Virginia, who was not involved with the study, told Ars. The authors “sort out, really, a lot of the ambiguity,” he said.

In the analysis, by Elliot Tucker-Drob of the University of Texas at Austin and Timothy Bates of the University of Edinburgh, researchers harvested data from 14 studies involving siblings, many twins. To be included in the analysis, the studies had to objectively measure intelligence and socioeconomic status of the kids. In all, the researchers captured data on 24,926 pairs of twins and siblings, which were fairly evenly distributed between the US and non-US countries, including Australia, England, Sweden, Germany, and the Netherlands.

When they separated the data by location, the authors found that the brute force of poverty in the US clearly pushed aside genetic influence on intelligence. But, the same relationship was not seen in any of the other countries.

That doesn’t mean that poverty is simply making US kids dumber, Turkheimer cautions. The situation is a little more nuanced. Imagine the siblings in the studies are flower seeds, he said. Those related seeds inherit genes that fix their IQ within a range of IQs that depends on their parents. Now, if you put those seeds in rich soil with all the nutrients and resources they could want, the flowers will grow to slightly different heights, based on genetic variation. But, if the seeds are grown in sandy, nutrient and resource-poor soil, those impoverished seeds will all be stunted. Basically, they’ll all turn out about the same height, he said.

So, Turkheimer said, kids in poverty all end up on the low side of their IQ window, losing that variation normally seen from genetics.

While the authors speculate that inequalities in educational and medical access in the US may beef up poverty’s effects, Turkheimer thinks school environments in particular may be to blame. He plans to follow up on the findings in his own work.

But, for now, researchers may at least be able to put to bed the debate of the Scar-Rowe hypothesis, Rob Kirkpatrick, a psychiatric and behavioral genetics researcher at the Virginia Commonwealth University, told Ars. “It goes a long way toward helping to explain the mixed replication record of the Scarr-Rowe interaction by showing that only studies from the United States, and not those from other Western nations, provide evidence for it,” he said.

Psychological Science, 2015. DOI: 10.1177/0956797615612727 (About DOIs).