Monday, November 26, 2007

Peer-reviewed nonsense: Active Parents Raise Active Children

The British Medical Journal - which is highly respected and has the 6th highest impact factor of all general medical journals - has just published an almost entirely worthless study on the effect of parental physical activity on the physical activity of their 11-12 year old children (Mattocks et. al., 2007). The study is worthless, in short, because it proceeds as if the entire field of behavioral genetics does not exist; the authors simply assume their conclusions are not confounded by genetic factors. It astonishes me that such fatally flawed article can get past peer-review in such a prestigious journal. That such an obvious confound as genetics can be overlooked is a testament to the continuing detrimental effect of the blank slate on modern science (Pinker, 2002).

First a bit more about the study itself. The authors used data from the Avon longitudinal study of parents and children, which collected (and is continuing to collect) a wealth of data from 14,061 families. The specific question addressed was which factors in the child's early life (defined as before age 5) influenced the objectively measured physical activity of the same children at ages 11-12. The authors collected the physical activity data with uniaxial actigraph accelerometers from 5,451 11-12 year old children in the Avon cohort and then looked at data collected when the children were aged 5 or younger for causal variables. In other words, the researchers wanted to know which early life variables predicted physical activity at age 11-12. The conclusion of the research was:
We have shown that children are slightly more active if their parents are active early in the child’s life. This suggests that encouraging physical activity in parents may also influence their children to become more active, with the added advantage that physically active parents are healthier (Mattocks et. al., 2007: 7).
So, in other words, active parents socialize their children to be active themselves. (It's clear the authors are thinking in terms of socialization, something the following quotation perhaps illustrates a bit better: "in our study, maternal activity during pregnancy... was positively associated with physical activity in the children. It is unlikely that this is due to biological factors in utero but is more likely that physical activity during pregnancy is a marker for later maternal physical activity and that this in turn influences children’s physical activity" [Mattocks et. al., 2007: 6].)

A slight problem...

Children share 50% of their genes with each parent, and since all human behavioral traits are heritable (the so-called First Law of Behavioral Genetics, Turkheimer, 2000), genetic factors are always possible confounds when relating parenting style (or other parental behavior) to outcomes in the children. As Turkheimer explains:

It is no longer possible to interpret correlations among biologically related family members as prima facie evidence of sociocultral causal mechanisms. If the children of depressed mothers grow up to be depressed themselves, it does not necessarily demonstrate that being raised by a depressed mother is itself depressing. The children might have grown up equally depressed if they had been adopted and raised by different mothers, under the influence of their biological mother's genes (2000: 162).
The exact same problem holds for the Mattocks study: one can't simply assume parental physical activity (or lack thereof) influences children to be active (or inactive) because it's possible that sedentary children inherit sedentary genes from their sedentary parents and active children inherit active genes from their active parents. Or, to put it differently, the fact that the physical activity of parents when the children were young is correlated with the children's degree of activeness later on simply does not constitute evidence of a socialization effect.

To be clear, I'm not claiming children are not socialized in this way; my point is we cannot tell one way or the other from the data presented because it fails to distinguish between the relevant causal hypotheses. I really hope I've somehow been daft by missing how the authors controlled for genetic factors. The alternative is that a leading medical journal published an article that is scientifically illiterate, that overlooks obvious possible confounds and that is thus worthless in terms of deciding what causes 11-12 year old children's degree of physical acitivity. Frankly, that I've made a mistake is far more palatable to me.

(See also: ScienceDaily's report on this research).

Mattocks, C., Ness, A., Deere, K., Tilling, K., Leary, S., Blair, S., & Riddoch, C. (2008). Early life determinants of physical activity in 11 to 12 year olds: cohort study BMJ, 336 (7634), 26-29 DOI: 10.1136/bmj.39385.443565.BE

Turkheimer, E. (2000) "Three Laws of Behavior Genetics and What They Mean," Current Directions in Psychological Science, 9(5): 160-164.

Pinker, S. (2002) The Blank Slate: The Modern Denial of Human Nature (London: Penguin).


  1. Yes, it looks pretty odd to me too. They clearly say repeatedly that the data "suggests" a conclusion with which it is certainly consistent, but in the absence of any consideration of the most obvious other player on the field. Of course the two possible influences are not mutually exclusive.

    We'd really need something like a twins reared apart study, or a study with some kind of intervention (paying parents in a sub-group to be more active) to start teasing out the empirical detail. We sure don't need ignoring behavioral genetics.

  2. I think you're exactly right - their data is consistent with their conclusion, but some sort of intervention (or a twin study) would be needed to determine what the causal mechanisms are. Ignoring behavioral genetics is a Bad Thing.