Äldre bröder är smartare än yngre visar en stor ny norsk studie som publicerats i tidskriften Nature. Det skiljer hela 2,3 IQ poäng i medeltal. De små skillnader yngre och äldre bröder får uppleva har gett en förhållandevis stor skillnad. Studien tyder alltså på att miljön har stor inverkan på IQ.
The mean IQ of first-born kids was just over 103, second-borns just over 100, and third-borns about 99, they found. But if a child's elder sibling had died, leaving him or her to be raised as first-born, their IQ lept up to match the top scores of 103. Likewise if both of two elder siblings had passed away, these third-born children had IQs matching that of first-borns, they report in Science1.
And it isn't just that big families have less smart kids, and small families have bright ones. In a separate analysis, to be published in Intelligence, the researchers show that the trend holds true for distinct pairs of siblings in their study group — even within single families, older siblings are on average smarter. "There can be no confounds in this type of study, and so the theory of spurious associations has effectively been refuted in one fell swoop," says Frank Sulloway, an expert on birth order and intelligence from the University of California, Berkeley.
Läs gärna min intervju med professor Bjarne Fjeldsenden som fördjupar diskussionen om arvet och miljöns betydelse för IQ.