Big babies, big heads, big IQ, and thoughts on eugenics.

By Nestor Lopez-Duran PhD

I spent the weekend helping my sister work on her dissertation. She is at MIT studying the role of eugenics in urban planning and architectural design in South America. So most of my weekend was spent reading about how in the early to mid 1900s  scientists, physicians and public health scientists in particular, promoted the underlying idea that the way to improve our society was to promote desirable populations. In most instances, their actions were appalling by todays standards (such as determining desirable and non-desirable immigrants based on morphology), but often the line that separates old eugenic ideas and todays theories was much more difficult to see. Was their effort to isolate people with communicative diseases so that they dont contaminate the superior healthy population that much different than some of todays practices? Or for example, was the French Lamarckean version of eugenics, which believed that populations could be improved by manipulating the environment even in the presence of undesirable genetic traits, that much different than our current gene by environment interaction research?

The tone has definitely changed (no current mainstream scientist speaks in terms of improving particular populations over others -aka explicit racism), and the underlying intentions have also changed from promoting desirable populations to alleviating suffering and improving health among all. Yet, I cant help but to think back to the old french members of the Museo Social when I read about intelligence research. I am not in any way suggesting that the authors of the study Im about to describe promote eugenic principles. Instead, I bring this point simply to open the discussion about those difficult to spot lines between current science and the early twentieth century eugenic thoughts that drove most of medical research at the time.

So what am I talking about?

In the latest issue of Pediatrics a team from Universities in Singapore, Australia, Canada, and New Zealand, reported the findings of an epidemiological study on the association between birth size and later IQ. Birth weight has been associated with cognitive performance in that children who are born significantly underweight (such as premature kids) tend to have cognitive deficits (ranging from simply lower IQ to severe deficits) when compared to kids that are born within the normal birth size range. Less is known however, whether variations within this normative range, such as variations in weight, length, or head size, also affect the kids future intellectual capacity or cognitive performance.

The authors studied a cohort of 1979 Asian (Singapore) children who attended mainstream schools and were participating in a large longitudinal study of risk for Myopia. Birth data were abstracted from the kids medical charts. IQ was measured using the Ravens Progressive Matrices test when the kids were in 2nd to 5th grade. The Ravens is a test of non-verbal reasoning which has been used as a proxy for intelligence (more on this later).

The results:

  1. Greater body length, body weight, and head size were all significantly associated with increases in IQ scores.
  2. For every 1Kg (2.2lb)  increase in body weight, the model predicted a 2.19 increase in IQ score. For every 1cm increase in length the model predicted an increase of 0.37 points in IQ
  3. IQ was not associated with gestational age
  4. An analysis of a sub-sample of siblings showed that the taller (at birth) sibling had significantly higher IQs that the shorter sibling.

In sum, the data seem to suggest that prenatal factors affecting body weight likely impact brain developmental processes that are associated with later cognitive performance.

Sorry Im being highly introspective in this post, but as I type Im wondering why this article made me think about the eugenic issues I read this past weekend. Maybe Im also reacting to Richard Nissbett wonderful lecture at the American Psychological Society two weeks ago regarding the role of culture and environment on IQ.  So I will end this post early by simply addressing three issues: 1) Im confused about the distribution of scores on their IQ measure and the norms used to reach those scores. Either this was not a representative population sample, or the norms used were outdated leading to significantly higher IQ estimates than expected in the population. Please note that even the low birth weight and short siblings had higher IQs than expected in the general population. 2) It is not quite accurate that the Ravens is highly correlated with IQ. In fact, the authors used a citation of a study conducted in 1956 that used measures that have been obsolete for decades. Instead, most recent research with newer IQ scales show that the Ravens progressive matrices tap only at some aspects of IQ and their IQ estimates are not highly correlated with full IQ scores obtained from current full batteries. Finally,  IQ in early to middle childhood is highly fluid and not a strong predictor of later IQ. Thus, I would love to see the developmental trajectory of these kids and examine more applicable outcomes such as educational attainment, etc.

The reference: Broekman, B., Chan, Y., Chong, Y., Quek, S., Fung, D., Low, Y., Ooi, Y., Gluckman, P., Meaney, M., Wong, T., & Saw, S. (2009). The Influence of Birth Size on Intelligence in Healthy Children PEDIATRICS, 123 (6) DOI: 10.1542/peds.2008-3344