News stories have been recently circulating around the web claiming that the most important genes for a child's intelligence come from his or her mom.
According to these reports, which seem to uniformly derive from this blog post, fathers play a relatively minor genetic role in determining a child's "intelligence quotient," or IQ, which is a score people get from taking a standardized test. Scientists use this score as a proxy for intelligence because it's measurable.
While there isn’t a lot of strong evidence either way, the idea that mothers' genes are the main determinant of their children's intelligence is most likely not true. Recent studies on the genetics behind intelligence (here, here and here, for example) point to many genes -- possibly thousands spread across our DNA and bequeathed to us by both our parents -- as affecting IQ.
X Doesn't Mark the Spot
Part of the argument that mom and not dad is mostly responsible for kids’ IQ is based on some older research that suggested many key genes related to intelligence are on the X chromosome. As you may recall from high school biology, biological males have one X and one Y chromosome; biological females have two Xs.
What you might not remember, though, is that sons can only receive an X from their mothers; they must get their Y from their fathers.
So the argument for mom-derived intelligence is that key IQ genes are on the X -- thus, boys must get their intelligence from their moms.
Of course even if this were true, daughters would still get a mix of their mother's and father's IQ genes, as males do give their female children an X.
But even the claim that male children inherit their intelligence from their mothers is dubious. That's because none of the recent studies have found key genes for intelligence on the X chromosome.
In fact, despite some studies suggesting that around half our IQ comes from the genes we inherit, the latest research has not been able to locate any genes which by themselves have a significant effect on intelligence.
The best explanation for not being able to identify the specific genetic origins for intelligence is that each of the hundreds or even thousands of genes contributing to intelligence have only a small effect on overall IQ.
So your final “genetic” IQ is the sum of all of these small effects, and each is simply too minimal to recognize in the kind of genetic studies that have been done to date.
Dad Data Missing
In the viral blog post mentioned above, one of the studies cited as proof of maternal determination of intelligence compared the IQs of 12,686 people between the ages of 14 and 22 to the IQs of their mothers.
The post reports that offspring IQs varied from maternal IQs by only about 15 points on average.
The study is attributed to the Medical Research Council Social and Public Health Sciences Unit at the University of Glasgow. We couldn't find it, so sent the unit an email inquiry.
Geoff Der, a senior research fellow at the unit, wrote back. He said the post appeared to be referring to this study on the effect of breastfeeding on intelligence, for which he was a co-author. (As it turns out, the data used for the study came from the U.S.)
The study does say that "maternal IQ has the largest independent effect" on the IQ of children, with factors such as maternal education, age, family poverty and birth order "all making independent contributions for most outcomes."
But Der says the study didn't measure paternal intelligence as a factor, because that data wasn't available. So for all we know, there could be an even greater correlation between the IQs of children and fathers. Der said the post's assertion that kids' IQs correlated to mothers' IQs within 15 points could not have come from his study.
However, even if IQs do track more with maternal intelligence, this might not be so surprising. Intelligence isn't just a matter of genetics -- environment plays a huge role, too. This is especially true in the first five years of life, when our brains are being molded by our circumstances. And traditionally, mothers have had more of an influence on children's' home environments (though this is changing, of course).
Adding to the environmental argument, this study, looking at the effect a father's age has on childrens' intelligence, said: "Factors such as parental education, socioeconomic status and number of, particularly older, siblings may play an important role in accounting for paternal age-AH4 associations." (AH4, like IQ, is an intelligence test.)
And of course if this effect turns out to be genetic and not environmental, then it shows that dad's genes can have a meaningful impact on his kids' intelligence.
The final argument for mothers determining a child's intelligence is based on an interesting set of mice experiments done between 20 and 30 years ago. One interpretation of the results from these experiments is that a woman's genes contribute more to the cerebral cortex or the thinking part of the brain, while a man's genes contribute more to the limbic system, which controls some of our more primitive functions like appetite, sex and aggression.
But while mice are often used as precursor subjects for human trials, the results don't always translate.
So where our level of intelligence comes from is hard to pin down. It could be that mothers or fathers play a bigger role. Right now, we just don't know.
What we can say is that proper nutrition and a stimulating environment can do wonders for everyone's mind, no matter what genes mom and dad pass down.
As usual, we are more than our genes.
Jon Brooks contributed to this post.