Research shows that both nature and nurture – genes and environment – play a role in shaping intelligence. Now, a new review finds that in the US, the effect of genes on intelligence varies with social class, but this is not the case in Western Europe or Australia.

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A long-standing theory in behavioral genetics is that poverty limits our ability to reach the intelligence potential set by our genes by reducing opportunities.

The study, published in Psychological Science, is the work of two psychological scientists, Elliot Tucker-Drob, an associate professor of psychology at the University of Texas at Austin, and Timothy Bates, professor of individual differences in psychology at the University of Edinburgh in the UK.

A long-standing theory in the field of behavioral genetics is that while our genes decide how much potential intelligence we have, we also need opportunities to express that potential in order to realize it – and poverty and socioeconomic disadvantage likely reduce such opportunities.

However, research in this area has so far given inconsistent results, making it difficult to argue whether this theory bears out in practice.

To try and resolve this, the two researchers pooled and re-analyzed data from 14 independent studies that looked at how the interaction between genes and childhood socioeconomic status related to intelligence and academic achievement test scores in the US, Western Europe and Australia.

The analysis only included studies that had used objective measures of intelligence and assessed family socioeconomic status in childhood.

Another important feature of the analysis is that it only included studies of pairs of siblings and identical twins so that the researchers could better separate the genetic from the environmental influences.

Altogether, the pooled data covered 24,926 pairs of twins and siblings from the US, Australia, England, Germany, the Netherlands and Sweden.

The researchers found that the interaction between genes, socioeconomic status and intelligence varied according to the country in which the participants lived.

The results indicate that the idea that genetic influence on intelligence depends on socioeconomic status appears to be true only of the US.

In fact, in the case of the Netherlands, the evidence points to an opposite effect.

The researchers suggest the distinction between the US and other countries could be due to differences in the experience of social disadvantage.

For example, Western Europe and Australia have more robust social welfare and health care programs, and these may buffer the effects of poverty.

Prof. Bates suggests a key question for future research will be to find out what it is that helps a society “break the link between social class and the expression of genetic potentials for intellectual development.” He concludes:

Once such characteristics are identified, they could inform policies directed at narrowing test score gaps and promoting all of the positive consequences of higher IQ, such as health, wealth and progress in science, art and technology.”

According to the US Census Bureau, in 2014, the overall poverty rate in the US was 14.8% – that is 46.7 million Americans living in poverty. However, the poverty rate that year for children under the age of 18 was 21.1%.

To determine who is in poverty, the US Census Bureau uses a set of money income thresholds that vary by family size, composition and ages of the members. There are 48 possible poverty thresholds that are updated every year to reflect changes in consumer prices.

Earlier this year, Medical News Today learned of a study published in the journal JAMA Pediatrics that suggests poverty produces structural brain changes and worse assessments of academic achievement in children. The researchers examined MRI scans of children in the US from different socioeconomic backgrounds and found regional gray matter volumes were 8-10% below the developmental norm in those who fell below the federal poverty line.