Is obesity a social contagion?


With rates of obesity in Australia only marginally behind the United States and tracking at the same pace, mathematical and social modelling on the projection of obesity rates in America is sobering reading for Australians.

The most recent statistics on the weight and health of the Australian population paints the grim picture of one in four adults classified as obese (defined as a body mass index above 30 kg/m2). When overweight is added to this, 63% of adult Australians are likely carrying more weight than what is good for them. These rates have been consistently rising for the last three decades and do not appear to show any signs of slowing.


A research team from Harvard University has applied complex mathematical modelling, derived from long running diet and health studies, to determine how obesity rates could look like in the United States in the future. As Australia closely matches and tracks the United States for obesity rates, and has a similar standard of living, then forecasts from this modelling would have currency for Australians.

The bottom line is that obesity rates will likely reach a peak of 42% of the population within 40 years. This result on its own is probably not so surprising, but the interesting finding that came from the research was the factors most likely to predict if a person would become obese.

The more friends a person has who are obese, the greater their own chance of becoming obese is. To put some hard numbers to the finding, each adult was found to have a 2% chance of becoming obese in any given year. But for every obese friend a person had, their own risk of becoming obese increased by 0.5 percentage points. So someone with a social circle of six obese friends would have a 5% risk of becoming obese themselves.

Having lots of thin friends though did not lower the risk of becoming obese, leading the researchers to propose the novel concept of obesity being like an infectious disease carried amongst a person’s social network.

Social networks are not the cause of obesity, but it appears they can help to propogate it. Living in an environment where overweight is the norm may make it harder for a person to see the health issues right in front of them from the accumulating kilos.