A while ago I wrote about the way some people think and talk about intelligence as fixed, innate, knowable, and implying something about a person’s worth. A fairly recent academic essay I came across challenges at least the first two of those claims.
Bruno Sauce and Louis Matzel have a paper in the Psychological Bulletin called ‘the Paradox of Intelligence’ which aims to reconcile two apparently competing themes within the intelligence literature.
The first of these themes is that researchers have reliably found intelligence to be very highly heritable. What does ‘heritable’ mean, I hear you ask – well, that’s largely the subject of the paper.
Heritability is a measure of the variation in a trait – in our case, general intelligence – that is due in some way to genetic differences. One important source for measures of this ‘heritability’ of a trait is through twin studies. Researchers look at pairs of twins who are either monozygotic (i.e. identical) or dizygotic (fraternal); twins of both kinds are assumed to share very similar environments; identical twins also share essentially 100% of their DNA, while fraternal twins share 50% like regular siblings. If you measure their respective IQs and find that identical twins are more similar than fraternal ones you can calculate the portion of the variation in IQ related to variation in genes.
Many such twin studies have been done and this proportion attributable to genes is very high: up to 0.8 (or 80%) in some cases. Twin studies have plenty of weaknesses, but Sauce and Matzel say that’s not relevant to their argument. They accept that the heritability of intelligence is high.
But at the same time as we see all this evidence of the high heritability of IQ, there are other lines of scientific inquiry that show intelligence can change for a particular person, and in pretty big ways.
The most famous of these is the Flynn Effect, named for James Flynn who observed and wrote about it in 1984. IQ tests are standardised so that the average score is always 100, but Flynn’s analysis had shown that the standard had been steadily increasing over time. Between 1932 and 1978 the average IQ rose by 13.8 points, or in other words, if someone who scored 100 on an IQ test in 1978 went back in time to 1932 and took the test then, they would have gotten a score of 113.8. There certainly isn’t enough genetic change in a couple of generations to explain such a jump. A similar increase has been observed in many countries around the world (though the effect appears to be receding).
Other striking evidence comes from adoption studies. For example, very poor children in Korea were adopted into much richer families in the US. The children who grew up in the US were subsequently found to have higher IQs than their peers who stayed in Korea. The effect was greatest for those children who had been malnourished before adoption, gaining between 10 and 40 points.
And there are other subtler examples: the rate at which intelligence declines with age has slowed down; and children in certain kinds of childcare programs show increased IQ, though the gains do not last when the children return to the academic environment they otherwise would have had.
So there seem to be these two opposed currents in the research – intelligence is both highly heritable and highly malleable in certain circumstances.
To solve this riddle we must return to what ‘heritable’ actually means. People might instinctually hear this word and think something like, ‘determined by genes,’ – and implicitly – ‘by genes alone.’ If intelligence ‘is genetic’ it cannot be changed, and the life consequences of intelligence, academic achievement, income, health etc. must be inevitable.
Here is where Sauce and Matzel come in. Basically, ‘heritability’ is a statistical construct that includes everything that involves genes, but not necessarily genes independent of the environment. Any and all of the interplay between genes and the environment get subsumed into the measure of heritability.
And this interplay can be incredibly complex, with feedback loops and correlations, gene-influenced actions and environmental reactions, starting from before birth and accumulating over a lifetime.
Consider a child who shows some innate musical ability which their parents encourage with music lessons, so that the child’s ability grows. The difference between this child and another without the innate ability will end up much larger than the initial genetic difference is responsible for, i.e. the difference in their ability if neither had had any music education. But all of this difference statistically comes under heritability even though the actual genetic difference is small.
I will add another example here, a point not discussed in the paper. We could imagine that these two children have the same potential ability, should each see it fully developed in an ideal environment, yet they differ genetically in terms of when the ability first shows. If the one, exhibiting their ability sooner, receives the most encouragement, while the late bloomer becomes dejected they may end up as adults with different measured abilities. Again, this difference appears as part of heritability, but it would be a mistake to think that because it involves genes, it must be inevitable.
But it gets more complicated. The child’s parents are also more likely to have this above average ability and so are more likely to have had a musical education themselves. This may then have made them more likely to give their child a home environment that develops musical abilities. This is an environmental effect, but one that is correlated with genes and so again it ends up bundled with heritability.
We don’t really have experimental methods to separate the independent genetic effect from gene-environment interactions. But that hasn’t stopped some people from overinterpreting heritability to try and justify their preferred policies. So keep a healthy scepticism if you hear someone claim that a complex psychological phenomenon is determined by genes.