In a discussion on the rise of mental health issues at Core Economics, Paul Frijters touches on the increase in obesity over the last 50 years.

One can basically out of hand reject the excuses most individuals give for their problems as being the reason. The rate of increase rules out any reasonable role for genetics. The fact that the poor suffer more from obesity, whilst it is cheaper to eat less and whilst food has always been cheap for the rich, rules out any obvious effect of the lower price of food or the availability of fast-food. The sustained increase over a long time rules out any story depending on some major current crisis. Like it or loath it, but it is clear that one must look at ‘cultural factors’ to have a hope of understanding what is going on.

A big hint comes from cross-national differences amongst rich countries, where things like wealth and food affordability don’t differ much. As you can see here, the Anglo-Saxon countries, and then particularly the US, stands out. Whilst a third of adults in the US are now obese (with about 25% of Australian adults), only 4% of Koreans and Japanese are such, and in the more egalitarian Northern European countries (Sweden, Norway, Holland) rates are below 10%. The same holds for Italy and France, though rates in those countries too are quite a bit up from what they were 50 years ago. So your one major clue is that there are major unexplained differences over countries.

I’ve heard this “it can’t be genetics” argument from a few people recently. And in some respects it is right. Clearly, the genes in the population have not changed substantially over the last 50 years. However, to dismiss genetics in trying to understand obesity is ignoring an important piece of the puzzle.

First is the high heritability of obesity - both before and during the increase in obesity of the last 50 years - usually measured in the range of 55 to 85 per cent. This level of heritability exceeds that measured for most behavioural traits (although it is in the realm of heritability for intelligence). This suggests that both before and after the increase in obesity, genetics plays a substantial role in who is obese. As the environment has changed (such as the “cultural factors” that Frijters alludes to), people of different genotypes have responded in different ways.

Another pointer to the role of genetics is in the high levels of obesity and obesity-related diseases in some populations, particularly indigenous populations with a limited history of agriculture. Pacific islanders, with almost no history of grain based agriculture, have the highest rates of obesity in the world. American Indian and Alaskan Natives (as a group) have the second highest level of obesity in the United States of any major ethnic group (behind people of Pacific Island origin). In Australia, aboriginals and Torres Strait Islanders are twice as likely to be obese as Caucasians. The costs of obesity, such as Type 2 diabetes, also tend to higher for these groups.

Frijters’s point on the response of the rich and the poor to food prices also has a hint of genetic factors. These groups are not the same. The rich and poor differ in average levels of IQ, willpower, conscientiousness and a host of other traits with a genetic component (even if you don’t believe there is a genetic component, you still should not treat them as the same). So when we ask why obesity is higher among poor people (or less educated people), we should ask what role those traits and their underlying genetic influences play. When faced with the same choices, they are likely to select different options.

These traits might also be relevant for cross-country comparison. Should we be surprised that East Asian countries where populations have higher measured IQ, lower rates of time preference and higher savings rates also have lower rates of obesity? (Obviously, on a cross-national basis, this is not the complete explanation. For example, East Asians in the United States have higher levels of obesity than their counterparts still in East Asia, although they are obese at rates lower than Caucasians and other ethnic groups).

We should also not be too quick to dismiss price. It is not only absolute price that matters, but also relative price of different food types. As argued by Rob Brooks, Steve Simpson and David Raubenheimer, simple carbohydrates have never been cheaper relative to protein. If you are price sensitive, you may shift consumption towards simple carbohydrates. As someone who tends to avoid simple carbohydrates, I can also attest that a large part of the relative price of food is the search effort in finding a low carbohydrate option.

The reason this matters also has an evolutionary basis. Eating food is not a simple “eat calories and feel full” process. Different foods create different responses in appetite. Brooks and his colleagues base their argument on the protein leverage hypothesis, which is a hypothesis that humans have a stronger propensity to regulate protein intake than they do for other non-protein calories. Humans eat until we satisfy out basic daily protein need. If the food we are eating has low protein content, we need to eat more before hitting that satiation point. These extra calories are what make someone obese. Trends in carbohydrate, protein and fat consumption in the United States over the last 40 years offer support for this argument.

Arguments such as the protein leverage hypothesis also have interesting implications for any arguments about the willpower of the obese. Someone eating a diet high in simple calories would need more willpower to constrain their calorie intake than someone on a high protein diet.

While Frijters points to the cross-national differences as a major clue to why obesity has increased, the above suggests that within country and cross-population differences will also be useful. The cultural changes that have resulted in the increase in obesity play out in different ways depending on who the person is. Genetics is clearly not the only factor that should be examined - look at the Anglosphere compared to Northern European countries - but any cultural explanation will need to accord with the evidence that the cultural changes do not affect all people equally.