The influence of genes on intelligence varies by social class in the United States, but not in Western Europe or Australia, according to a new study that compiled results from 14 previous studies.

Researchers tentatively attributed the findings—published in the journal Psychological Science—to the effect of stronger social safety nets in those other countries.

Past research suggests that both genes and environment shape intelligence. A popular hypothesis is that genes confer a potential intelligence, but whether this potential entirely bears fruit depends on whether the environment is supportive and nurturing—or poor and disadvantaged.

Some studies have supported this view; others haven’t.

In the new work, psychologists Elliot Tucker-Drob of the University of Texas at Austin and Timothy Bates at the University of Edinburgh in the U.K. conducted a “meta-analysis,” a study that combines data from previous studies.

The pair said they used all available published and unpublished studies meeting specific conditions. The studies had to contain an objective measure of intelligence, a measure of participants’ family socioeconomic status in childhood, and participants that varied in relatedness (i.e., siblings versus identical twins) to allow for statistically disentangling genetic and environmental influences.

Tucker-Drob and Bates analyzed data from a total of 24,926 pairs of twins and siblings who had participated in studies independently conducted in the United States, Australia, England, Sweden, Germany, and the Netherlands.

“The hypothesis that the genetic influence on intelligence depends on socioeconomic status was not supported in studies outside of the U.S.,” said Tucker-Drob. “In the Netherlands, there was even evidence suggestive of the opposite effect.”

The study showed no evidence that other factors influenced the results, such as age of testing, whether the tests measured achievement and knowledge or intelligence and whether the tests were of a single ability or a composite cognitive measures.

The researchers suggest that the difference between the United States and other countries might be due to stronger health and social welfare programs in Western Europe and Australia, which might reduce poverty’s negative effects.

A key question for future research will be to identify which aspects of a society hinder the expression of genetic potential for intellectual development, Bates said. Understanding this “could inform policies directed at narrowing test score gaps and promoting all of the positive consequences of higher I.Q., such as health, wealth, and progress in science, art, and technology.”