Although obesity can't actually spread like an infectious disease, contact with other obese people does influence the likelihood of becoming obese yourself, Hill said.
After analyzing 40 years of data from 7,500 people in the Framingham Heart Study, researchers found the average person has a 2 percent chance of becoming obese in any given year, due to personal factors such as an unhealthy diet or lack of exercise. That chance increases by 0.5 percent for every obese family member, friend or coworker a person has regular contact with, Hill said.
An obese person has a 4 percent chance of losing enough weight to no longer be considered obese in any given year, the study said.
"Maybe it's mimicking behavior or adopting similar lifestyles, or changing what you consider to be a normal and acceptable weight for yourself based on the weight of people around you," Hill said. "It's most likely to be some social influence."
Hill has used the same mathematical model to look at how positive and negative emotions can spread among people. She said she plans to use the model to look at how social contact can affect other behaviors.
The study was published today (Nov. 4)