The movie Idiocracy suggests that intelligence is hereditary; this idea has also been perpetrated by German author Thilo Sarrazin, who claims that due to the higher fertility of Islamic immigrants and their allegedly lower intellect, Germany was dumbing down.

My question: Is IQ (mainly) a genetically inherited trait?

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    One mistake of Idiocracy is that they tell you the (average) IQ is going down. By definition an IQ of 100 is the average, i.e. the tip of the Gaussian curve, so would stay constant. Ans you should take everything what Thilo Sarrazin wrote and said very sceptically. Apr 8 '11 at 8:44
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    I know that I should take it skeptically. That's why I'm asking here :)
    – Lagerbaer
    Apr 8 '11 at 14:05
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    A second mistake of Idiocracy is that IQs have been rising for almost a century - it's called the Flynn Effect (see indiana.edu/~intell/flynneffect.shtml ) and gains in IQ testing vary from 5 to 25 pts per generation in all examined countries. This is a strong indication that IQ tests don't actually measure intelligence, but something related to intelligence, since no-one seriously believes the average IQ of the 1920s was in the severely retarded range. Apr 28 '11 at 14:04
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    "Few scientists argue that people weren't dumber then" really? Could you give an example of a scientist who believes the Flynn Effect reflects a real change in population intelligence rather than a change in cultural response to testing, formal education, and timing requirements? The only mainstream approach that seems to credit the Flynn Effect as a real intelligence change rather than a testing/cultural artifact is the nutrition hypothesis, which I think has some merit, but not to the point of 1900 Americans actually having an average IQ of 70 (not actual data, just illustrative). Jul 22 '11 at 2:27
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    A complete different topic: To my knowledge Thilo Sarrazin didn't talk about the past (Germany was dumbing down) but about the future. You need to know that he isn't a scientist of genetics, biology, medicine or psychology, but an economist, banker and politician. Feb 8 '12 at 0:18

From http://www.iq-tests.eu/iq-test-Genetics-versus-environment-400.html

The role of genes and environment (nature vs. nurture) in determining IQ is reviewed in Plomin et al. (2001, 2003). The degree to which genetic variation contributes to observed variation in a trait is measured by a statistic called heritability. Heritability scores range from 0 to 1, and can be interpreted as the percentage of variation (e.g. in IQ) that is due to variation in genes. Twins studies and adoption studies are commonly used to determine the heritability of a trait. Until recently heritability was mostly studied in children. These studies find the heritability of IQ is approximately 0.5; that is, half of the variation in IQ among the children studied was due to variation in their genes. The remaining half was thus due to environmental variation and measurement error. A heritability of 0.5 implies that IQ is "substantially" heritable. Studies with adults show that they have a higher heritability of IQ than children do and that heritability could be as high as 0.8. The American Psychological Association's 1995 task force on "Intelligence: Knowns and Unknowns" concluded that within the White population the heritability of IQ is "around .75"

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    No. It doesn't mean you get "half your IQ" from genes and have to learn the rest. It means that half the variation of your IQ is due to variations in genes.
    – Lagerbaer
    Apr 8 '11 at 14:07
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    The first statement would mean that half of your IQ points are due to genes and half of them due to upbringing. The second statement means that half of the points you are above or below average are due to genes and the rest due to upbringing. So, very simplified, if I had an IQ of 120, it means that 10 of the 20 points I'm above 100 are luck in the genetic lottery, and 10 of them are my environment.
    – Lagerbaer
    Jul 22 '11 at 17:45
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    This can't possibly be right. How would this explain the 4 and 5 year old kids who are tested with very high IQ scores? THey grow up and keep those same high IQ scores. They don't get dumber because of some flaw in their upbringing.
    – Dunk
    Feb 7 '12 at 23:15
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    In case you don't quite understand how IQ is measured, it is simply a number based on your relative score to your peers. At 4 and 5 years old these children have a high IQ RELATIVE to other 4 and 5 year olds, not relative to everyone in the population. So no, children would not get "more stupid" as they get older, that does not make any sense, but lacking a proper education their peers (everyone else) could get smarter while they don't improve as much.
    – Asaf
    Feb 9 '12 at 4:20
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    @Dunk: If they're inhaling lead paint dust on a regular basis (or other environmental toxins that cause accumulative neurological damage) or suffering from malnutrition, then yes, they could be getting dumber over time because of their upbringing.
    – endolith
    Aug 23 '12 at 16:21

While I find the above answer accurate, I think it needs further clarification because it answers a question that is different from the one that was asked.

The question was: "Is IQ (mainly) a genetically inherited trait?" The answer related not to how heritable IQ is, but to how heritable individual differences in IQ are. The point of confusion is that 'heritability' has a very special meaning in the literature, as it relates not to how strongly a trait is expressed, but to how much differences in a trait are carried over from generation to generation:

The concept of heritability plays a central role in the psychology of individual differences. Heritability has two definitions. The first is a statistical definition, and it defines heritability as the proportion of phenotypic variance attributable to genetic variance. The second definition is more common "sensical". It defines heritability as the extent to which genetic individual differences contribute to individual differences in observed behavior (or phenotypic individual differences).

The original question actually can't be answered based on current research, because the research revolves around individual differences. I want to offer some further explanation on the the answer that was given, which tells us how heritable individual differences in IQ are. I don't have many references to back me up because it is just a discussion of the meaning of correlation, but in the context of intelligence, it has been briefly discussed here

When a trait is heritable, there is a correlation between how much it is expressed in parents and their children. This means, if we were to take all the parents and rank them based on IQ, and then take all the children and rank them, the 'parental rankings' and the 'child rankings' would be similar: the parents with the highest IQs are most likely to have children with the highest IQs, and so on. The important thing here is that both these rankings are made within one age group only. This means, in theory, that all children could have an IQ vastly above or below their parents, and yet the correlation would persist. The size of the correlation won't change if parents and children have different IQ scores on average, as long as the rankings don't change within each group.

The Flynn effect is a known instance of children consistently getting better IQ scores than their parents in each successive generation, with no effect (I believe) on how heritable IQ is. It is similar with height: height is a highly heritable trait, but kids nowadays are taller than their parents more often than not. Or, to give an example adapted from the paper I linked above, imagine you are looking at differences between the length of stalks of wheat in a field. Then add fertilizer, observe that the stalks have all grown by 1/3 of their original height, then look at the differences in their lengths again. They are likely to be preserved, i.e. their rankings won't change. In the context of IQ, as long as an environmental effect persistenly affects a whole generation, IQ can remain highly heritable, while the scores of parents and their children can differ significantly. This is true of adopted kids (also mentioned in the first link), whose IQ correlates with that of their biological parents and not with their adoptive parents, but is still closer to adoptive parents'.

The take-home message is that the size of a trait, and how correlated that trait is to something, are two independent measures. So, even if individual differences in IQ are highly hereditary, this does not tell us how similar IQs of two related people are.

  • What you're saying, essentially, is that you don't believe in the real-world relevance of any twin studies.
    – Fizz
    Oct 14 '17 at 2:33
  • @Fizz No... I'm saying IQ tells you something about your relative position in a group, rather than being an absolute measure of some quantity. (This is how it's defined in the literature, methodologically speaking. You can then speculate that there is a unique quantity of something in the brain driving this, but that is not how the measure is derived.) And when you ask 'is my position in a group heritable', you can see that you need more than just your individual genes to answer that question. This is what I was trying to unpack / unpick.
    – Ana
    Oct 16 '17 at 19:46
  • @Fizz - more specifically, I'm saying that knowing the amount of heritability does not necessarily help you predict IQ. In reality it often does, of course. But I think that seeing situations where it doesn't, helps understand the subtleties involved in measuring and interpreting this type of data.
    – Ana
    Oct 16 '17 at 20:02

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