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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.

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