Like most Diseases of Affluence, obesity is the result of the interplay between genetic and environmental factors. Polymorphisms in various genes controlling appetite and metabolism predispose to obesity when sufficient calories are present. As of 2006 more than 41 of these sites have been linked to the development of obesity when a favorable environment is present. The percentage of obesity that can be attributed to genetics varies, depending on the population examined.

Strangely, obesity rates vary considerably in the United States amongst different ethnic groups, which many social scientists attribute to institutionalized socioeconomic oppression of certain minority groups (such as African-Americans and Latinos), who can only afford cheap, heavily processed and pre-packaged food high in fat and calories. However, while socio-economic conditions undoubtedly play a role in rising obesity rates, many scientists believe that poverty doesn’t account for the whole story.

A hotly contested alternative theory purporting to explain obesity amongst certain ethnic groups thrifty gene hypothesis postulates that certain ethnic groups may be more prone to obesity in an equivalent environment. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would be more likely survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies.

Some experts don’t agree entirely with the “thrifty gene” theory – saying that poverty, stress due to racism, and poor nutrition (especially during pregnancy) also result in children being born predisposed to diabetes. Most experts agree that genetics alone does not determine the risk factor of obesity; but research does suggest that “thrifty genes” predispose people to hold on to food (sugars), making them more likely to develop type II diabetes. It is also common for indigenous groups to be poorer and have less access to healthy foods than the general population, so it is difficult to separate genetic from environmental factors.

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