Social Contagion
A research article titled “Infectious Disease Modeling of Social Contagion in Networks” uses the epidemic modeling we discussed in class to analyze obesity with a few interesting caveats. One difference between obesity and typical epidemics (infectious diseases) is that obesity is a behavioral phenomenon that can be transmitted by non-social mechanism in additional to social mechanisms.
Traditional epidemics are modeled by classifying individuals into 2 groups where they either have the disease (infected) or do not (susceptible), and the disease may be spread to someone without the disease upon contact with someone who is infected. The rate of this transmission and the rate of recovery are regarded as constants, uninfluenced by other factors. However, the probability of getting obesity is not a constant because it is increased by social interactions with people who are obese; researchers determined that the rate of obesity is 2 percent and increases by .5 percent with each obese social contact. Recovery, however, is independent of obese contacts at 4 percent. The qualification with recovery is that in traditional susceptible-infected-recovered (SIR) disease modeling, recovered individuals become immune to the disease and are classified as recovered, but obesity can occur multiple times in an individual’s life so this study did not make that assumption. Instead, it used the susceptible-infected-susceptible (SIS) model, reclassifying recovered individuals as susceptible.
By adjusting traditional epidemic models to account for the specific circumstances surrounding obesity, researchers were able to predict a long-term obesity prevalence of 42 percent and accurately reproduced the actual historical prevalence of obesity. Recently, both spontaneous infection and transmission rates of obesity have increased while the rate of recovery from obesity has stayed the same, suggesting that the obesity epidemic stems from more people becoming obese and not obese individuals failing to recover successfully.
One fascinating insight from this research was the idea of contagion in one direction. The fact that the chance of obesity was increased with social contact to those who were obese but the chance of recovery was uninfluenced by social contact means that gaining weight is contagious but losing weight is not. Researchers conducted additional research to find that smoking cessation was contagious even though smoking itself was not.
References:
http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1000968