The Educational Gap for Infants

Genes for mental ability get a boost from socioeconomic status in a study of baby twins.
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Family income and parental education begin to make a difference in a child’s mental achievement as early as infancy, according to new research in behavioral genetics that advances the ill effects of poverty to late infancy.

In a study of 750 pairs of infant twins from a range of places, family incomes and ethnicities, a team of researchers led by Elliot Tucker-Drob of the University of Texas at Austin found that 2-year-olds from affluent families were scoring moderately higher than their lower-income peers on tests of mental capacity. The tests included pulling a string to ring a bell, putting three cubes in a cup, matching pictures, sorting pegs by color, and repeating vowel-consonant combinations.

Previous studies have shown that socioeconomic status has an influence on the expression of genes for mental ability across the life span, beginning at age 7. The UT research, in collaboration with the University of British Columbia and the University of Virginia, suggests that the interaction between genes and family income begins even earlier, effectively creating a performance gap between poor and wealthy children by the age of 2.

“The literature says genes matter more and more as people get older,” said Tucker-Drob, an assistant professor of psychology at UT. “But there’s an underrepresentation of low-income participants in most existing twin studies. In our more diverse study of twins from both poor and wealthy families, we found that genes start to play a role by 2 years of age, but only for children being raised in wealthier families.”

Previous research estimates that in the general population, 50 percent of mental ability is inherited and 50 percent is attributable to the environment.

Using methods from sociology and demography to help define the nature-nurture interaction, the UT team studied identical twins, who share 100 percent of their genes, and fraternal twins, who share 50 percent, across a range of family incomes. If identical twins were more similar in their test scores than fraternal twins, it stood to reason that genes were contributing. To calculate socioeconomic status, the researchers asked parents about their education, occupation and income.

At the age of 10 months, the study showed, socioeconomic status was not related to mental ability. By two years, however, the scores for mental ability were moderately higher for twins in affluent families with educated parents. Also in the wealthier families, the identical twins tended to be more similar to each other in their test scores than the fraternal twins. The researchers concluded that socioeconomic status was giving expression to the genes for mental ability in children being raised in wealthy families, but not in poor families.

The UT team did not attempt to identify what specific behaviors led to the disparity, but previous studies have suggested that high-income parents spend more time with their children and are able to draw on more resources to fit their educational needs.

In poor families, Tucker-Drob said, “people tend to have fewer books, are more overworked and stressed out, and their nutrition is not as good.

“Our findings suggest that children from poor families are already starting off in kindergarten, on average, at a disadvantage in their early cognitive skills.”

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