Parents have been squabbling over the answer since the beginning of humanity. So – is it our mothers that we owe our intelligence to?

He got his mother's eyes, but what about his intelligence?
He got his mother's eyes, but what about his intelligence?

This question originally appeared on Quora: Is it true that intelligence is inherited from the mother? Answer by Drew Smith, Ph.D. in Molecular, Cellular and Developmental Biology from the University of Colorado

The first rule regarding new studies of the inheritance of intelligence is to be very suspicious of all studies of the inheritance of intelligence.

The history of these studies is not a happy one. Stephen Gould wrote a whole book (The Mismeasure of Man) detailing all the ways in which biological research has been co-opted in the service of racism, misogyny, homophobia and economic repression. Intelligence research, not surprisingly, has always been one of the worst offenders.

The whole field suffers from foundational weaknesses: we cannot really define intelligence, and cannot separate it from culture. Psychologists do indeed have definitions of intelligence but (surprise!) these definitions tend to emphasise skills at which psychologists excel: verbal fluency and manipulation of abstract symbols, for example. A cynic might add to this list a willingness to believe that results from experiments on affluent Western undergrads can be generalized to the rest of humanity.

That said, there is little doubt that there is a heritable component of intelligence. Whatever forms intelligence might take, it is ultimately about problem-solving, and the best problem-solvers are the ones who live to reproduce. Although the idea of genes "for" intelligence is a fallacy, there are many genes whose function affect intelligence. The survival value of different alleles of these genes is highly dependent on environment, not least the societal and cultural environment in which they operate. Few, if any, of these alleles make their bearers "more" intelligent. They are instead likely to affect the forms which intelligence takes.

It is not remotely plausible that all of the genes which affect intelligence are inherited from the mother, but that is not what the linked research claims - rather, it claims that a preponderance are. There are a few mechanisms by which this could happen:

  • A number of genes that affect intelligence could be located on the X-chromosome. Since males have only one X, and inherit it from their mothers, their intelligence might be more closely linked to their mothers than to their fathers.
  • Genetic imprinting[1] [2] [3] can cause genes inherited from one parent to be preferentially suppressed. This seems to be what the new research is claiming. Although this is plausible, I'd wait for a few more confirmatory studies before accepting it as fact.
  • Mitochondria are inherited (almost) exclusively from the mother. Though mitochondrial genes might not seem to be genes "for" intelligence, brains are voracious consumers of metabolic energy - which is supplied by mitochondria. Better mitochondria are likely to translate to better brain function.

A lot of the research cited in the article claims to show that children's intelligence is more closely correlated with that of their mother than their father. Well, duh. So long as mothers are the primary caregivers, they are also the primary architects of their child's environment during the period of critical brain development. Of course smart mothers tend to raise smart children. Raising children to be good problem solvers is itself a problem to be solved, and mothers with better brains are likely to do a better job of it.

The genetics of intelligence is one of the LaBrea Tarpits of science. It is covered with the sticky tar of subjectivity, and is still struggling to climb out of the tar and on to a firm objective footing.

Footnotes

[1] Genetic Imprinting and X Inactivation

[2] Imprinting, the X-Chromosome, and the Male Brain: Explaining Sex Differences in the Liability to Autism

[3] Genomic imprinting on the X chromosome: implications for brain and behavioral phenotypes.

Source: TRTWorld and agencies