"Long before it's in the papers"
December 16, 2015


Whether genes affect intelligence may depend on class, country

Dec. 16, 2015
Courtesy of the Association for Psychological Science
and World Science staff

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.”

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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 potentials 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.”