Is intelligence genetically inherited?

14 February 2010


Can people have intelligence passed down genetically? My parents are both fairly knowledgable and can absorb information easily, so since I seem to take after them, is this nature or nurture?


Chris - Ah, The nature and nurture debate!

This is a really hard question to answer for a number of reasons, one of which is how do we assess and appraise IQ (intelligence quotient). There's a guy called Jim Flynn who was working in New Zealand, at the University of Otago, and he's credited with noticing what's now dubbed the "Flynn effect." He points out that if you look at IQ test results that go back over many, many years, the average IQ has been rising by about three IQ points every decade. This means that if a hundred is the average IQ (an average person in a population with average IQ has an IQ of a hundred), that means that if you take off three decades worth of improvement, people would've been in the mentally subnormal category a few years back, which obviously, they weren't! He makes the point that it's the way in which we measure IQ which has to be considered as well. So some people just don't trust the IQ test.

But if you assume that we do, what proportion of the IQ that you or I have is because of the way our parents brought us up, and what amount is due to the genetic legacy that we inherit from those parents? Well there's one quite neat way of looking at this and you can probably guess what it is: to look at twins. Nature has blessed us with a set of natural clones, identical twins, and also a set of non-identical twins which grow up usually in the same environment as each other, but they don't have the same DNA in common. So you can compare the two and that's exactly what researchers have done in the past. There's a guy called Paul Thompson who is at UCLA in America; he published a beautiful paper in 2001 in Nature Neuroscience. What he did was to take 40 twins - 10 identical pairs of twins and 10 non-identical pairs of twins - and carry out brain scans on them.

They compared the fine structure of the brain in both groups of twins and they found that identical twins had 95 to 100% similarity in the fine structure of their brains. But the non-identical twins were much less similar.

Then you ask, "well, in that setting, how similar were their IQs?"

When they did simple IQ tests, they found that, for the identical twins, there was a significantly stronger relationship between their IQs than there was between the non-identical twins. So this shows that there's something to do with the way in which the brain is wired up and connected that definitely contributes to how intelligent we feel we are or that people measure us as being. So, the best guess at the moment is that there is some general factor - some people dub it "G" - which is what gives you your intelligence, and this seems to have a genetic effect. This has, superimposed on it, an environmental and nurture effect. So, in other words, you have a genetic legacy which is then manifest according to how nature impacts on it. So if you have a good educational upbringing and a good genetic legacy, then you probably will fulfil your genetic potential. That's not to say everyone who has a very high IQ is going to fulfil their genetic educational potential because they may just not get educated. So I think that the best guess is that it's about 50/50.

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