‘Nature v nurture’ IQ debate continues

Breastfeeding in the first few months of life can “boost children’s IQ by seven points”, the Daily Mail and other newspapers reported. The effect only occurs in those who carry a particular genetic variant, but The Independent said that “most babies could potentially benefit from breastfeeding in terms of a raised IQ” as the gene variant is present in 90% of the population.

The research is a study of how environmental and genetic factors interplay to affect our intelligence. It raises the debate on “nature versus nurture” but does not produce any conclusive evidence. There are many other factors involved in our development and it is currently not possible to say that those who have a particular form of this gene will benefit more from being breastfed than those who do not.

However, breast milk has many established health benefits, and it is these that should be promoted in the cause of breastfeeding, rather than whether or not it may make your child more intelligent.

Where did the story come from?

This research was conducted by Avshalom Caspi and colleagues of Kings College London, Duke and Yale Universities of the US, and University of Otago in New Zealand. The study was supported by National Institute of Mental Health, Medical Research Council, and the Health Research Council. The study was published in the peer-reviewed scientific journal Proceedings of the National Academy of Sciences.

What kind of scientific study was this?

This was a cohort study designed to test the theory that intelligence is determined by both genetic and environmental factors. In particular, the researchers looked at how the link between breastfeeding and intelligence is modified by having a variant of a particular gene (FADS2). This gene encodes a protein involved in the body’s processing of certain fatty acids. Previous studies have found that these fatty acids accumulate in the brains of babies who are breast-fed during the first few months of life.

The researchers looked at two cohort studies. The first study from New Zealand involved 1,037 people in the Dunedin Multidisciplinary Health and Development Study, who were born around 1972 and then followed until the age of 32. The children were enrolled at age three, and the mothers were interviewed to find whether or not they had been breast-fed. The children were tested with a standard scale to determine their IQ at ages 7, 9, 11, and 13. DNA samples were obtained from them when they were adults.

The second study, conducted in the UK, involved people from the Environmental Risk Longitudinal Twin Study, who were all twins born in 1994 and 1995. They were enrolled in 1999 to 2000 when 1,116 families with same sex five year old twins took part in home visit assessments. Whether or not the child was breastfed had been established by postal questionnaires to the mother when they were two years old, and their IQ was then tested at age five. Again, DNA samples were obtained from the children.

The researchers examined how the link between breastfeeding and IQ was affected by the presence of certain variants of the FADS2 gene, to see whether genetic effects could modify environmental influences. They looked at single variations in nucleotides, the building blocks of DNA and RNA, at two specific sites within the FADS2 gene. Attempts were made to adjust for other factors that could have an influence such as social class, mother’s mental ability, and growth restriction in the uterus.

What were the results of the study?

The New Zealand study found there was a 5.6 point difference in IQ scores between breastfed and bottle fed children, while the UK study found a 6.3 point difference. The overall average IQ score of those who were breastfed was higher.

In both the New Zealand and UK cohorts, there was an interaction between which variant of nucleotide they had at one of the sites in the FADS2 gene and the effect of breast milk.

Those children who carried a specific gene variant had greater IQs if they were breastfed than if not. There was no significant effect in the IQ of children who did not have this gene variant. This association was not affected by social class, mother’s IQ, or the variant that their mothers had. They also found that the variant was not associated with a greater likelihood of being breastfed, or with better growth in the womb.

The UK study (but not the New Zealand study) found that variation in the nucleotides at a second site in the FADS2 gene also had an effect on breastfeeding and IQ.

What interpretations did the researchers draw from these results?

The researchers conclude that children who are carriers of a particular variant of FADS2 obtain more benefit from breast milk than those who do not, suggesting that ”genetic variation in fatty acid metabolism moderates breastfeeding effects on children’s cognitive development”.

What does the NHS Knowledge Service make of this study?

Our intelligence is not controlled by one factor alone, and is affected by many genetic and environmental factors. In this study, the effects of breastfeeding have been assessed in the context of variations in a single gene that is involved in the breakdown of fatty acids in milk. The results of this relatively small and preliminary study should not be considered as conclusive proof of the relationship between the FADS2 gene and the effect of breastfeeding on IQ; substantially more research is needed before this can be confirmed.

Some confounding factors, which may distort the true relationship between variables, have been considered. However, many hereditary factors, and important environmental ones, such as the type of schooling, have not. For the majority of us, our own genetic make up and that of our children are unknown to us, and even if it were we are currently unable to bend it to our will.

For all of these reasons, women should not worry that any benefits that breastfeeding may have on intelligence may be cancelled out by their genetic makeup. One headline stating that “Breastfeeding is good – if it is in the genes” may be rather poorly misinterpreted.

However, breast milk does have many established health benefits, and it is these that should be promoted in the cause of breastfeeding, rather than whether or not it may make your child more intelligent.