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Getting Science Right: Is IQ Nurture or Nature?

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The influence of the environment on children’s IQ.

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IQ predicts, to a certain extent, your social and occupational status, your educational and job performance and your health as an adult. But, other than the popular believe, IQ does not only statically predict your life course. IQ is in turn also influenced by that life course.

Scientists already reported that children’s IQ is still sensitive to the environment. New research now suggests that this especiallly counts for children with a high IQ. How does that work? We ask Angela Brant, leader of the study recently published in Psychological Science.

So, as opposed to adults, children’s IQ can still be influenced by the environment?

Yes, children’s IQ is more malleable than adults, although evidence suggests that some environmental influence persists for adult IQ. My research in particular suggests that this change comes at the end of a sensitive period for IQ development during adolescence.

And you found that that period is shorter for children with a low IQ?

Yes, it seems that the shift towards an adult pattern of genetic and environmental influences on IQ happens earlier for children with lower IQ. This transition happens in late childhood/adolescence, but the exact timing is related to IQ score. This finding supports earlier theories based on developmental neuroscience findings.

How did you test that?

We tested this using a sample of just less than 11,000 twin pairs of differing ages across the lifespan (raised together), and confirmed the findings in a sample of twins, biological siblings and adoptive siblings tested at various ages between 1 and 16.

We compared the resemblance in IQ scores between identical twins (who share all their genetic material)  to that of fraternal twins (who share 50% of their varying genetic material on average) in childhood, adolescence and adulthood. This showed how much of the differences was accounted for by genes, how much by environments shared by siblings and how much by individual-specific environments.

Then we examined whether the resemblances were dependent on IQ score at each age and thus were able to determine whether the switch between childhood and adulthood patterns of influence was dependent on IQ score.

What kind of environmental influences should we think of?

My study does not look at specific environments, it is more a survey of the collective importance of the environment on variation in intelligence. But we can think of various potential sources of variance, like characteristics of the home environment, perhaps SES and schooling. Prenatal influences are also a potential candidate that shouldn’t be forgotten.

Are these environmental influences just as likely to increase as well as decrease IQ?

The assumption is that the environment can be both a positive and a negative influence , but this assumption is not explicitly tested in this research.

Furthermore I wouldn’t necessarily use the phrase “increasing or decreasing IQ”. The child’s cognitive capacity is always increasing. Environments can promote that growth to varying degrees during the sensitive period.

How can genetic influence increase in importance over time? Aren’t these already present at birth?

There are two main ways in which genetic influences can increase over time: Firstly, when the magnitude of other influences decreases, the influence of genes relatively increases. The second is that new genetic influences can become important, additional to the already-acting genetic variation. Now, like you say, genetic variants are present at birth. The expression of such genetic variants can change over the lifespan, however. This can happen for many reasons, in response to environmental circumstances and as part of the normal developmental process.

As you write in the report, your findings can not be explained by the idea that individuals with a high IQ could be shaping their own environment and hereby positively influencing their IQ?

I would not discount the potential for individuals to shape their own environment, I certainly believe that humans self-select according to their interests and also provoke certain responses from others by our behavior. I also would not discount the effect of this process on IQ. I state in the article that this can not explain the main novel result of my study: that individuals with high IQ show an extended sensitive period. This is because that explanation would require that children with high IQ begin to self-select their environments later than children with lower IQ, which seems to be counter-intuitive. This suggests that there is another process at play that leads to increasing genetic influence. I suggest that this is the limitation of environmental malleability caused by the closing of a sensitive period in brain development.

Do most scientists agree with your conclusion or are their groups of scientists that think very differently about the influence of nature and nurture on IQ?

The broad pattern that I describe is well-supported by the extant literature, although this study is the first to demonstrate differing developmental trajectories of change according to IQ, and the first to explicitly test for a sensitive period in IQ development.

There are disagreements among disciplines, however, in terms of what these patterns of results mean for explaining the causes of individual differences in IQ and in terms of practical implications for promoting positive outcomes.

Photo: Flickr, whiteafrican
Source: Brant AM, Munakata Y, Boomsma DI, Defries JC, Haworth CM, Keller MC, Martin NG, McGue M, Petrill SA, Plomin R, Wadsworth SJ, Wright MJ, & Hewitt JK (2013). The Nature and Nurture of High IQ: An Extended Sensitive Period for Intellectual Development. Psychological science, 24 (8), 1487-95 PMID: 23818653

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