The german politician Thilo Sarrazin made the claim that 80% of intelligence is inherited. Other parts of his statement included that we germans are getting dumber on average because all those foreigners are coming to germany, so he clearly has an axe to grind.
I’m wondering now if we even know with some certainty how much intelligence is influenced by genetics. Is there a significant correlation between the IQ of the parents and that of their children?
Even if there was a very strong correlation between IQ of parents and IQ of children, that wouldn’t mean IQ was inherited.
It might be that higher IQ levels make one more likely to succeed financially, and that living in a higher income home makes a child more likely to have a higher IQ.
If intelligence were not inherited at all, then there could be no physical traits (such as brain structure) which affects it, and I think that is mildly unbelievable. So the answer to the question in your title is yes. The answer to your real question (nature vs. nurture) is a lot harder to figure out.
According to wikipedia, various twin studies have shown that about 50-90% of the varience in IQ is heritable, so the 80% number is defensable.
Those studies seem to be comparing populations raised in somewhat similar environments though. I suspect the vast unwashed third-world masses your German politician is so worried about would have a significant extra variance due to things like malnutrition, exposure to lead and other pollutants, etc. that the German population as a group probably hasn’t suffered as much from in recent history.
It’s hard to study because, for the most part, children are raised by their biological parents. There’s certainly a correlation between the intelligence of parents and children, but how much of that is genetic, and how much is due to the parents teaching their children well? You can, of course, study children adopted at birth, or the like (or, ideally, identical twins separated at birth), but it’s hard to get enough of them to get good statistical results.
I looked at the Wikipedia article on Heritability of Intelligence, and it seems 0.75-0.85 is somewhat of a consensus. That is significantly higher than I expected.
Even if the number is defensible, it does not validate the other stupid stuff he said. I also did not ask just to refute him, although I expected that he invented most of his statistics. The occasion just made me curious.
That is not how heritability studies work. They typically use twin or adoption studies so the home environment is not a determining factor.
To the OP: Yes.
How well are adoptive studies controlled for economic status? I’d suspect that low income folks would be more likely to put children up for adoption, and high income folks more likely to adopt them, and it wouldn’t be unreasonable for intelligence (or IQ) to be correlated with economic status.
The aforementioned twin studies compare identical twins (who share ~100% of their genes) to fraternal twins (~50% genes shared), and often to siblings (~50% shared) and nonrelated individuals (~0% shared).
Identical twins and fraternal twins are raised similarly (granted parents are more likely to do things like dress identical twins the same) so the environments are shared. Nevertheless you will find that a huge portion of the similarity in identical twins is due to genes. You can do this for many traits including IQ
All this suggests that IQ is highly heritable. Whether IQ = intelligence is another matter.
Intellegence can also be affected by the environment during gestation. Twin studies can’t be used to pick out truly heritable effects from congenital, but not genetic, ones.
I was also going to post something myself about the fact that genetically identical twins pretty much always have at least one shared environment: the womb. Intelligence is quite possibly correlated with maternal nutrition, meaning any correlation you see may not be from the heritability of intelligence but the fact that both children developed in the same womb. While it might be possible to control for, I highly doubt there have been enough identical twins carried by mothers other than their genetic ones to make a useful study.
Some twins share amniotic sacs while others are separate. Those environmental effects can also be measured.
And you can at least somewhat control for prenatal conditions by comparing identical twins to fraternal twins, and fraternal twins to non-twin siblings.
If you go by I.Q. tests, neither Germans nor the residents of any other country (assuming that test most children for I.Q.) are becoming less intelligent. Look up the Flynn Effect. Over the entire history of I.Q. testing, the average I.Q. has risen by around 3 points each decade for any country where I.Q. testing has been common enough to be able to notice a trend.
(In case anyone is going to quibble about this, yes, I know that the average I.Q. is going to be 100 no matter what. What I mean is that if you take the children of age X in year Y, who will have an average score of 100 on the I.Q. test given in year Y designed for children of age X, they will have an average score of 103 on the I.Q. test given in year Y - 10 designed for children of age X. I.Q. tests are periodically recalibrated because of this.)
I would just like to add that the Flynn Effect appears to have ended in many countries. For example, British teenager’s IQ peaked in the 1980s, and has slightly declined since then. In Norway, tests given to conscripts showed scores increasing for 1950 to the mid '90s, and then they plateau or decline after that. Cite is the last part of this article.
I honestly had never even considered the possibility that intelligence wasn’t inherited. I though it was a given.
Very recent history, given how long it has been since The Wall fell, and the conditions under which Eastern Germans were living.
Of course, and like the OP said, the really serious question about that politician’s affirmations is “can you prove that those immigrants have lower IQs than the locals?” (running the tests in the immigrants’ first languages; tests run in a second language do not count unless you can also prove that the people being tested are perfectly fluent in this second language).
To be exact, it looks like the Flynn Effect has reached a plateau for some groups. It seems that in the most developed countries and in the richest parts of other countries the cultural changes that caused the Flynn Effect have had as much effect as possible. It looks like the effect continues in other countries and poorer parts of some countries. In any case, there’s no proof that in developed countries like Germany the genetic possibilities for I.Q. are being pulled down by the immigrants. It’s more likely that the average I.Q. for the immigrant groups will eventually rise to the level of the country that they have immigrated to.
Oh for crying out loud. Yes, intelligence is largely heriditary. But the claim that an influx of foreigners would lower the general IQ is ridiculous. In fact, the opposite is much more likely.
Think about it, what kind of people are economic immigrants? They are the kind of people, compared to their countrymen, who show the most initiative, the most courage and determination. They leave their homes and countries to go work in another country another culture. Would you do that?
Such immigrants are quite often also the ones with the best marketable skills. You don’t go to another country if you don’t believe your work will fetch a higher price there. Now, in countries where education is not as advanced and omnipresent as here in the west, marketable skills will be highly correlated with IQ. A smart boy in a small rural village in Turkey might more easily become a kick-ass car mechanic then go to college, even when he has the IQ for it.
If only there were some kind of “final solution” to that problem…
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