GLASGOW, Scotland -- If smarter women are more likely to breast feed their babies, it only stands to reason that their children would inherit higher IQs, reported researchers here. It's nature, not nurture.
GLASGOW, Scotland, Oct. 4 -- If smarter women are more likely to breast feed their babies, it only stands to reason that their children would inherit higher IQs, reported researchers here. It's nature, not nurture.
Calling the higher IQs of children of breastfeeding women purely circumstantial, not a factor of the mothers' milk, the investigators reported an IQ advantage of four points in children who were breastfed, mostly attributable to brainy mothers.
The study, which is five times larger than any previous study on the issue, used data from the U.S. national longitudinal survey of youth that began in 1979, reported Geoff Der, M.A., M.Sc., of the Medical Research Council Social and Public Health Sciences Unit here, and colleagues, online in BMJ.
It included 12,686 young people aged 14 to 22 when first interviewed who were interviewed annually until 1994 and every other year thereafter.
Looking at breastfeeding alone as a contributor of babies' IQs, the advantage was small (0.52 points) and non-significant (95% confidence interval ? 0.19 to 1.23) once fully adjusted for a range of relevant confounders
The mother's IQ was a better predictor of whether she would breastfeed than race, education, age, poverty status, smoking, the home environment, or the child's birth weight or birth order. One standard deviation increase in maternal IQ (15 points) more than doubled the odds that a woman would breastfeed her child.
"Maternal intelligence is relatively overlooked as a potential confounder," the researchers wrote. "This is surprising given the heritability of intelligence and the known association of maternal intelligence with both the initiation and duration of breast feeding."
The study assessed children's IQs every other year using the Peabody individual achievement test with all scores standardized to a mean of 100 and a standard deviation of 15. Maternal intelligence was assessed with the U.S. Armed Forces qualification test segments for arithmetic reasoning, word knowledge, paragraph comprehension, and mathematical knowledge. Since 1986, other children of the participating women have also been assessed biennially. Preterm and low birth weight babies were excluded.
The unadjusted effect of breastfeeding was a 4.1 to 4.7 point advantage compared to children who were not breastfed though controlling for maternal IQ reduced this advantage by 71% to 75%, and adjustment for mother's education by 34% to 42%.
An analysis of only the children who were breast fed found a significant effect for reading comprehension (0.14 points, P=0.003) and Peabody individual achievement test total score (0.11, P=0.025) when mother's IQ was omitted. However, "all effects were small considering that the median duration of breast feeding is three months and the 95th centile is 14 months," the researchers noted.
Another analysis of 332 sibling pairs with different breastfeeding status and 545 with different durations of breast feeding also found no cognitive advantage for breastfeeding after controlling for confounding factors.
The researchers also conducted a meta-analysis of the available literature on intelligence and breastfeeding. Of the eight studies, those with the biggest IQ advantage for breast feeding "were those with smaller sample sizes and that controlled for fewer of the important additional confounders" including socioeconomic status, maternal education, and maternal age. Overall, there was an IQ advantage of 3.37 points controlling for maternal IQ but
no other confounders but "effectively no advantage" (0.16 points) controlling for maternal IQ and eight additional confounders.
"Studies that do not control for maternal intelligence will probably give biased results," the investigators wrote.
These findings come in the context of an extensive literature showing health, nutritional, immunologic, developmental, and psychological benefits of breastfeeding for babies and mothers. The American Academy of Pediatrics advocates breastfeeding as do other governmental and professional associations including the World Health Organization.
Der and colleagues noted that even if breastfeeding does not confer cognitive benefits, it is still ideal for the healthy growth and development of babies.
The authors cautioned against over-generalization of these studies. "The generalizability of the results presented here must be considered carefully," the authors wrote. "This study and the others included in the meta-analysis are all based on samples from developed countries: United States, Europe, Australia, and New Zealand. Generalization of the findings beyond these and similar societies would be unwise. We have also excluded premature and low birthweight infants for whom the effect may be different."