Action Potential

Genetic control of intelligence

No, this isn’t another post about Jim Watson; the title is just a shameless ploy to draw in the search engine crowd… It is relevant though, as you’ll soon see. I’ve been meaning to talk about this next topic for a couple of weeks now, but have been distracted with unexpected news stories, editorial discussions regarding the review process, etc. A recent study in PNAS has demonstrated a significant interaction between nature and nurture that influences intelligence.

It has long been stated that breast-fed babies perform better on IQ tests, suggesting that brain development is enhanced by this “nurturing” factor. The augmentation in development could arise from the particular fatty acids that are found in human milk, but not in cow’s milk or in baby formula. But in order to take advantage of those extra fatty acids, one must be able to metabolize and process them. This is done in part by an enzyme called FADS2, which contains various polymorphisms in the human population. Repeating the same study in thousands of children from both England and New Zealand, researchers determined that in order to take advantage of the potential benefits offered by consuming breast milk, the infant must have one copy of the “C” allele. Those infants with 2 copies of the “G” allele might as well have been fed formula, because the 7 IQ points conferred by breastfeeding was absent in the “GG” babies. About 10% of the population is thought to lack any copies of the C allele.

The study went on to rule out alternative explanations, such as gene-exposure correlation, intrauterine growth, social class, maternal cognitive ability and maternal genotype effects on breastfeeding and breast milk. These results really change the climate of the mini-debate exploring the benefits of breast feeding. Many have suggested that the advantages enjoyed by breast-fed babies may arise because those mothers take a different, more hands-on approach to child-rearing, as opposed to the mothers giving their babies formula and TV. Actually, an interesting follow-up study would be to test the language skills of breast-fed infants that watch Baby Einstein videos

Evolutionarily speaking, it is curious that the GG allele has stuck around. Natural selection should have weeded out this less advantageous polymorphism, with the greater intelligence of the C-possessing population allowing them to compete victoriously over their less-intelligent brethren. But, logically, with the GG genotype persistent at 10%, it must have conferred an as-of-yet unidentified reproductive advantage to those individuals, allowing this genetic combination to perpetuate. Remember, you don’t have to be a science geek / nerd to procreate (…wait a minute…of course!!! Intelligence can sometimes be a reproductive curse!!! Just ask the men at Caltech, where only 29% of the student enrollment is female…)

So is the nature vs. nurture debate dead? Are we stuck with another unsatisfying denouement in science where the answer is “somewhere in between?” Perhaps, but for those extremists amongst you who long for contrast and want nothing to do with the gray zone, you’ll hate this next story. Staying with the breast-feeding theme, it seems that first-time macaque mothers bias their milk composition in favor of their sons. Examining over 100 animals, researchers from UCLA determined that milk was richer and more nutritive when mothers nursed their sons, as opposed to daughters. Therefore, the maternal energy investment was greater for male offspring. This makes sense, since for males, reproductive output correlates with the strength and size of the animal, allowing it to successfully compete for more mates. Females inherit the reproductive status of their mother, a sort of macaque caste system. Therefore, the mother obtains a greater Darwinian advantage by massively investing her own resources in the nourishment of her sons, while little is gained from producing richer milk for daughters.

Since the milk consumed by the males has a much higher fat content, perhaps we can extrapolate from the other study and presume that brain development in the male macaque is also enhanced, leading to a slight advantage in their intellect. If so, then on which side of the debate is this last finding? Obviously nature (genetics) determines the sex, but infant gender influences the treatment by the mother, altering the nourishment (nurture). I guess once again, it falls somewhere in between. Oh yea, only if the monkey is not “GG”.

Caspi, A., Williams, B., Kim-Cohen, J., Craig, I., Milne, B., Poulton, R., Schalkwyk, L., Taylor, A., Werts, H., & Moffitt, T. (2007). Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism Proceedings of the National Academy of Sciences, 104 (47), 18860-18865 DOI: 10.1073/pnas.0704292104


  1. Report this comment

    colin said:

    The problem with linking this gene to nuture variation is the fact that formula is a very recent development and thus could not be a strong evolutionary influence. There would be very few babies raised sans breast milk in the evolutionary environment of early humans. At best this may have been an advantage for babies whose mothers died.

  2. Report this comment

    Noah Gray said:

    I think you missed part of the picture, Colin, and perhaps I spliced two different concepts together rather coarsely. My thought was that since all babies were breast-fed, and some babies could not reap the extra benefits from the nourishment, then those unfortunate individuals should have been placed at a disadvantage because of their genetic make-up. Natural selection usually gets rid of these individuals, removing the genetic variant in the process. However, since it has stuck around, this variant must play another role or confer an independent advantage, allowing it to perpetuate. We just don’t know what that potential extra advantage might be.