Intelligence “Inherited From Mothers”? Well Some Of It, Sure | Vol. 3 / No. 45.3

Photo: Flickr user Allan Ajifo, CC BY 2.0

So, I’m a feminist. As such, I belong to a number of feminist Facebook groups and follow a number of feminist blogs. And 99% of the time that’s great. I get a lot of useful news about issues that matter to me that way. But every now and then, I see something that makes me go… wait, what now?

Such was the case when I encountered a blog post from a site calling itself Psychology Spot, entitled “Did you know that intelligence is inherited from mothers?” And after reading the article, I’m pretty sure they mean “mostly or even entirely from mothers.”

“No,” I thought, “I did not know that.” And then: “Does anyone ‘know’ that?”

I was skeptical because (a) general intelligence is really hard to pin down, and because (b) I find it hard to believe that something so complex as “intelligence” could possibly be inherited from only one parent, especially in the cases of female humans since, you know, they have two X chromosomes, one from each parent. My interest being thus piqued, I took a closer look.

What I found was two basic arguments, paraphrased below, each one being a combination of some serious research and some serious reaching.

Argument 1: “Genomic Imprinting” means paternal genes are for feeling and maternal ones are for thinking

Some of the terminology was hard to pin down, which I think is probably a linguistic issue (I think English is not the author’s first language, and technical terms are hard to translate). Take the following passage:

At the basis of this idea there are those known as “conditioned genes”, that behave differently depending on their origin. Basically, these genes have a kind of biochemical tag which allows to trace the origin and reveals even if they are active or not within the progeny cells. Interestingly, some of these affected genes work only if they come from the mother. If that same gene is inherited from the father, it is deactivated. Obviously, other genes work the opposite, are activated only if they come from the father.

I’m not a trained scientist, just a skeptic, which means I’m used to Googling terms a lot. But a Google search for “conditioned genes” didn’t turn anything up. I kept searching, trying to describe this idea of genes inherited from the mother and father that are silenced depending on which parent they’re from, and what I ended up with is something called “genomic imprinting.”

Genomic imprinting is an epigenetic process (or processes) by which certain genes “imprint” (i.e. shut off) so that the other parent’s copy can do the work. It’s an important process, and imprinting malfunctions can lead to either under- or over-expression of certain parts of the human blueprint, resulting in conditions like Prader-Willi Syndrome and Angelman Syndrome.

After reading a half-dozen articles I think I’ve not only discovered the 1984 paper the author forgot to actually cite (despite referring to it as “one of the first studies in this area [that] was conducted in 1984 at the University of Cambridge”), “Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis,” but also come to understand the basis for the first argument the author is making.

As the author explains, when researchers studying imprinted genes wanted to see which did what, they quickly realized that just making embryos from just the mother’s or just the father’s genes went very poorly. Many of the mammalian genes affected by imprinting are apparently involved in the growth of the embryo, and so without the paternal genes which suppress certain maternal ones, the placenta doesn’t grow right. That’s right, dudes: the placenta regulating instructions are in your gametes (this actually gets into a theory called “Genetic Conflict Hypothesis” — which seems to be about males extracting everything they can for each child since they can always get a new mate, but females holding back so they can have more kids — but I might be mischaracterizing that so I’m just going to sidestep that for now).

Anyway, later the researchers made chimeras so they could keep mouse embryos alive long enough to test what imprinted genes went where in the brain, and they found that the paternal imprinted cells stayed pretty much out of the neocortex and striatum, and congregated in the hypothalamus, septum, preoptic area, and the bed nucleus of the stria terminalis, which wikipedia informs me “correlates with anxiety in response to threat monitoring.” (If you have access, check out an article called “Genomic imprinting and the maternal brain,” by Barry Keverne from Progress in Brain Research 133, it’s really informative.) The embryos with only maternal cells, meanwhile, ended up with a much larger telencephalon, which is the part of the embryonic brain that later turns into the cerebrum.

What does this mean? Well, according to Keverne in the article cited above,

“Such a finding emphasizes the importance of maternally expressed alleles in telencephalic development and the lack of importance of paternally inherited imprinted genes in these regions.” 

The emphasis above is mine, and explains how we’ll diverge from the Psychology Spot author’s claims. When they write that “researchers have not found any paternal cells in the cerebral cortex, which is where they develop the most advanced cognitive functions, such as intelligence, thought, language and planning” [emphasis mine], they’re making a pretty big mistake.

This is about the imprinted cells. This is a statement about the roles and origins of certain key genes in embryonic brain development. This says all sorts of interesting things about the development of the human brain from an evolutionary standpoint. But this does not mean there are none of the father’s genes in the cerebrum, and none of the mother’s genes in the limbic system.

It means that the genes that regulate the development of these areas are parentally linked, not that all of the genes involved are.

Argument 2: Intelligence is in the X chromosome so intelligence comes from mothers

Now, males may have more to thank/blame their mothers for than females, but we knew that anyway. When you only have one copy of the X chromosome to work with, you need all of those genes to work right the first time. That’s why there are a lot more X-chromosome genes that, when they break or malfunction, lead to mental impairment. Males only have the one X-chromosome, but they give that X-chromosome to their daughters, who have two. The author actually writes, “since women have two X chromosomes [they] are twice as likely to inherit the characteristics related to intelligence.” Tell me again where that second X chromosome comes from?

But even that doesn’t mean that intelligence is only, or even mostly, inherited from mothers — even in males.

In fact, one of the studies the author cites, “A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution?” is all about the processes by which natural selection could use an X-chromosome-based “gene for general cognitive ability” to drive human intelligence. And despite its (admittedly very cool) findings, it nevertheless concludes that “it should be stressed here that a healthy lifestyle of the mother during pregnancy, well-balanced nutrition, and intensive and responsible care by the parents are far more important factors in a child’s cognitive ability than its genetic make up.

And of course, this is part of the conclusion the author of the Psychology Spot article comes to as well, saying near the end that “it is estimated that between 40-60% of intelligence is hereditary,” and that the rest is “environment” and “stimulation,” but they continue to overstate the binary distinction between ‘paternal genes and emotion’ and ‘maternal genes and intelligence’ writing: “even if intelligence is closely linked to the rational thinking function, it is also influenced by intuition and emotions, that genetically speaking, are influenced by the contribution of the father.”

That’s a lot of confidence for not a lot of evidence, right there.

Anyway. It ends with the bizarre platitude, given the shakiness of the evidence, that “fathers should not be discouraged, because they also have much to contribute to the development of their children, especially being emotionally present.

From what I can see, even if more than half of the really squishy thing we think of as general intelligence weren’t environmental, fathers still shouldn’t be discouraged, not just because they can still be “emotionally present,” but because (a) there might still be genes linked to intelligence in the Y chromosome, and (b) they still have X chromosomes to pass on genes to their daughters (and grandsons), too.

So as I said above, I’m not a trained scientist, I’m just a practiced skeptic with access to journals. Maybe some of my assertions above are wrong, or I’ve misunderstood here or there. But from what I’m seeing, there just isn’t the evidence to conclude that “intelligence is inherited from mothers” and not fathers, too, as the article implies.

What do you think, readers?

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Richard Ford Burley is a human, writer, and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.

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