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Intelligence favors first-borns, study finds

June 21, 2007
Special to World Science  

Eld­est sib­lings score slightly high­er on in­tel­li­gence tests than young­er sib­lings, a study has found, but the dif­fer­ence seems due to family dy­nam­ics rath­er than bi­o­logy.

Chil­dren reared as the eld­est—even if they’ve lost one or two old­er sib­lings—have an in­tel­li­gence quo­tient, or IQ, that is high­er by an av­er­age 2.3 points than their young­er sib­lings, sci­en­tists said. 

Pet­ter Kris­tensen of the Na­tional In­sti­tute of Oc­cupa­t­ional Health and Tor Bjerkedal of the Nor­we­gian Armed Forc­es Med­i­cal Serv­ice, both in Os­lo, stud­ied stud­ied the birth or­der, IQ and family situa­t­ions of more than a quar­ter mil­lion 18- and 19-year-old Nor­we­gian draftees. 

IQ is a nu­mer­i­cal meas­ure of in­tel­li­gence that in­creases with great­er in­tel­li­gence, and with the av­er­age IQ set at 100. An IQ score is sup­posed to rep­re­sent a per­son’s men­tal age di­vid­ed by their chron­o­log­i­cal age, and mul­ti­plied by 100.

The study on birth or­der and in­tel­li­gence ap­pears in the June 22 is­sue of the re­search jour­nal Sci­ence. It’s an “el­e­gantly de­signed anal­y­sis” that may help lay to rest dec­ades of past de­bate on the sub­ject, wrote Frank J. Sul­loway of the Un­ivers­ity of Cal­i­for­nia, Berke­ley, in a com­men­tary in the jour­nal.

High­er IQ in first­borns could be due to the tu­tor­ing that the eld­er chil­dren usu­ally per­form, Sul­loway wrote, but fu­ture re­search could help re­solve this. 

Past stud­ies have pro­vid­ed con­fus­ing and mixed re­sults on the sub­ject of birth or­der and in­tel­li­gence, Sul­loway said. Part­ly, this is be­cause when chil­dren un­der 12 are tested, cu­ri­ous­ly, it’s the young­er sib­lings who score higher. 

In oth­er words, there’s a “ten­dency for IQ dis­par­i­ties by birth or­der to re­verse di­rec­tion as chil­dren get old­er,” he wrote. One the­o­ry to ex­plain this, he added, is that the ben­e­fi­cial ef­fects of tu­tor­ing take sev­er­al years to kick in. Un­til then, more im­ma­ture young­er sib­lings might ac­tu­ally drag down the eld­er’s in­tel­lec­tu­al de­vel­op­ment slightly.


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Children raised as the eldest sibling score slightly higher on intelligence tests than than their younger siblings, a study has found. But the difference seems due to family dynamics rather than biological factors, reasearchers added. Children reared as the eldest—even if they’ve lost one or two older siblings—have an intelligence quotient, or IQ, that is higher by an average 2.3 points than their younger siblings, scientists said. Petter Kristensen of the National Institute of Occupational Health and Tor Bjerkedal of the Norwegian Armed Forces Medical Service, both in Oslo, studied studied the birth order, IQ and family situations of more than a quarter million 18- and 19-year-old Norwegian draftees. IQ is a numerical measure of intelligence that increases with greater intelligence, and with the average IQ defined as 100. An IQ score is supposed to represent a person’s mental age divided by their chronological age, and multiplied by 100. The study on birth order and intelligence appears In the June 22 issue of the research journal Science. It’s an “elegantly designed analysis” that may help lay to rest decades of past debate on the subject, wrote Frank J. Sulloway of the University of California, Berkeley, in a commentary in the journal. Higher IQ in firstborns could be due to the tutoring that the elder children usually perform, Sulloway wrote, but future research could help resolve this. Past studies have provided confusing and mixed results on the subject of birth order and intelligence, Sulloway said. Partly, this is because when children under 12 are tested, curiously, it’s the younger siblings who score higher. In other words, there’s a “tendency for IQ disparities by birth order to reverse direction as children get older,” he wrote. One theory to explain this, he added, is that the beneficial effects of tutoring take several years to kick in. Until then, the more immature younger sibling might actually drag down the older one’s intellectual development slightly.