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I have heard and read on the Internet the disputed claim that mixed blood* children are smarter than their (non-mixed blood) parents.

The general idea of "hybrid vigour" is known as heterosis and Wikipedia explains:

It has been suggested that many beneficial effects on average health, intelligence, and height have resulted from an increased heterosis

Other examples of discussion of the claim:

These sources couldn't convince me about the matter.

Is there any evidence to support the claim, to falsify it, or to demonstrate that it is an unscientific claim?

*I interpret "mixed-blood" to mean an offspring of parents of different races; i.e. having different ethnicity, skin color and geographical roots in general. For example, having a Japanese mother and African father.

Related question: Is race a discredited scientific concept in biology?

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    I wonder if the Flynn Effect would make this true, regardless of what race the parents were. – user1873 Mar 31 '13 at 4:06
  • @user1873 wouldn't Heterosis be closer? – bummi Mar 31 '13 at 8:14
  • Send your ideas as an answer if you can prove something. – Persian Cat Mar 31 '13 at 10:56
  • @user1873: Good point. It would make more sense to compare to children of the same generation and to account for all the other confounding factors. (Are mixed-race couples more likely to be liberal? Socially outcast? Rich or poor? etc.) I can see this being a very difficult claim to support. – Oddthinking Mar 31 '13 at 14:33
  • Freakenomics had some interesting takes on this. – Chad Apr 1 '13 at 15:38
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TLDR: Not satisfactorily answered.

Edit: A new interesting study on this topic found runs of homozygosity to be associated with intellectual disability in autistic cases. It is to be seen whether this finding generalises to intelligence variation in the general population.

Heterosis (outbreeding elevation) has been proposed as a reason for the Flynn effect (secular rise in IQ scores) by Mingroni in 2004 and 2007.

However, the most-accepted principal cause for the Flynn effect may be that it is on measurement specifics (Wichert et al., 2004) and thus does not reflect a "real" latent increase.

There has been some studies on inbreeding depression that found decreases of intelligence. As this blogger says the field is not sexy, lots of the studies are old.

This is probably

  • because it's hard to find good data
  • there's lot of stuff to control for (for example people with a certain degree of genetic relatedness are excluded and the first sixteen+ principal components/population stratification are usually controlled in molecular genetic studies on quantitative traits like intelligence when trying to identify related genetic variants. One reason they do this, among others, is that they don't want to pick up on confounds such as culture, differences in education between families or ethnic groups etc.). There are molecular studies that explicitly look at heterozygosity though.
  • because it's politically hot territory, that has been claimed by self-styled "race realists" (JP Rushton, S Sailer etc.). Reading my opinion on these people would likely not be good for the eyes of children, but I think it's safe to say that their work is not held in high regard by the wider scientific community and they tend to cite disingenuously and jump to politically incorrect conclusions possibly for the sake of contrarianism.
    I didn't really get why they are so interested in inbreeding depression and outbreeding elevation, because it might, after all, prompt people to marry non-Whites to have smarter kids, but I think they're mostly using it to pile invective on cultures where first-cousin marriages are still more common.
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    @PersianCat I'm saying the question is probably not sufficiently well-researched to make it feasible to provide a good, balanced answer, especially considering the sensitivity of the topic. I think you'll find people more willing to jump to conclusions from certain political camps, but I'm not inclined to trust those much. – Ruben Apr 8 '13 at 11:55

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