Friday, 3 May 2013

Fat is an intellectual issue

As a general rule, social scientists should be restricted to three factors, and allowed to deal with four factors only when they have achieved public renown. In my own case, I try to restrict myself to two, because holding two ideas in mind is hard for a psychologist.

It is clear, from both epidemiology and casual observation of the British street scene, that Britons are getting fatter. The great fatness that was visible in the US has now spread here. Clearly, this has a simple, one factor explanation: people are eating more. A cursory look at human metabolism shows that a small amount of food goes a long way, unless you are doing heavy manual labour, where a moderate amount of food goes a long way. Replacing the calories expended by exercise takes about one minute. I can be precise about that. Yesterday my 600 meter, 23 minute morning swim used up 175 calories (and my exercise watch is probably being generous). My usual yoghurt contains 154 calories. Croissants? Why should I touch those? I am told that an almond croissant contains 504 calories, but that need not concern us here. Assuming normal exercise during the rest of the day, the maximum I will need is 2,500 calories. Yes, no more than 5 almond croissants, but let us drop the subject.

Let us search for the second factor. Why is it that some people are fatter than others? At this stage the “multifactorial” gang make their entrance: glands, genetics, advertising, international food businesses, supermarket display shelves, television ads, leisure, lack of local sports facilities, and a profound crisis of capitalism/consumerism/spiritual values. I agree to all of the above, but what I am asking for is a predictive variable measured when children were young, and then evaluated against fatness in middle age.

Such a second factor has been identified by Kanazawa (2013).  He has used a famous population sample: all those babies (n= 17,419) born in England, Wales, and Scotland during 1 week (03-09 March 1958).  This is a proper sample: none of your convenience sample of n=30 sort of rubbish. Kanazawa had been surprised that other researchers had argued that, once you corrected for education, there was no link between intelligence and subsequent obesity. It turns out that this finding is based on partial use of the intelligence data. He found that if you include the full set of intelligence tests at 7, 11, and 16, and measure weight at 51 years of age (oh, the power of population samples) then there is a strong relationship, such that even intelligence at 7 predicts weight at 51.

In a very interesting twist, the effect of intelligence on weight only becomes apparent when children leave home, and can control what they eat. It is the opposite of all the “you are conditioned by your family environment” theories. Freedom of choice leads to different life styles, mediated by intelligence.

At this stage you may wish to stop reading, because I have just shown you evidence that intelligence at age 7 predicts obesity at age 51, QED. Stay with me a while. Parental BMI also has an influence. Fatter parents have fatter children. How does one explain that? Is it that children ape their parents through some cultural modelling? Probably not, because as we have just learned, the effect happens long after children leave home. It may be that parents with low IQ are themselves more likely to grow up to be fat, and that they have passed on the genes for low intelligence to their children. Obesity is just a side effect. It’s that pesky problem of collinearity again, and again. Are children inheriting genes for intelligence or genes for obesity, or both? A proper genetic study (of the sort which has not really been possible until now) might elucidate the causal pathway. Furthermore, although the picture is clear, the IQ/obesity link is not all that strong, and other factors also have an influence. Perhaps, after a genome wide study, we can identify the three or perhaps even four factors which account for most of the variance.

In fact, the whole issue is almost as much of an intellectual challenge as to whether one should eat an almond croissant.


  1. "this has a simple, one factor explanation: people are eating more." Setting aside your own implication that it's because people don't do the sort of heavy manual labour that consumes calories, you still miss the point: why are they eating more? It's no use saying that it's because food is cheaper - food has always been cheap if you're rich enough, but the rich have not always been fatties. Fat rich people were always, were they not, remarked upon?

    So what is it?

  2. In evolutionary terms it makes sense to eat whenever possible, since historically that always led to survival when food was hard to obtain. You have to be bright to understand that we are in a novel situation: too much food too easily available, such that obesity threatens life. The contingencies have changed. Now even the wealthy are thin (formerly they were often fat) because they value life over illness, and can show their status in other ways

    1. Where's your evidence for these assertions?

    2. Here is a good summary of the argument from an evolutionary perspective.

      King, B. M. (2013). The modern obesity epidemic, ancestral hunter-gatherers, and the sensory/reward control of food intake. American Psychologist, 68, 88-96.

  3. Oh dear, oh dear, I'm a great sceptic about arguments based on our ancestors' habits when, in truth, we know so little about them.

    "You have to be bright to understand that we are in a novel situation: too much food too easily available, such that obesity threatens life." Oh go on with you. Look at those descriptions of the dinner tables of the Victorian Haute Bourgeoisie and Aristocracy.

    "Now even the wealthy are thin ... because they ... and can show their status in other ways" - forgive me for altering the thrust of that quotation a little but isn't that exactly how they show their status nowadays, by being thin? This is reminiscent of the swings of its being fashionable to be pale, then sunburned, then pale again: but is it any deeper than that?

    My contention is, and I'm not just teasing here, that I've not really heard a persuasive account of why so many people have suddenly become so grossly fat (a far better term than "obese" which has been hijacked by the medical trades), or, rephrased, why so many people don't find their appetite readily assuaged. It's somewhat reminiscent of the question of why, in just a couple of generations, the Dutch have become so tall. They weren't when I was a wee laddie; now they are. What's up? (Insincere apologies offered for pun.)