Fat-fueling factors were taken into consideration. For instance, the researchers made sure the effect wasn't a case of "birds of a feather flocking together." Body measurements were taken throughout the study period, showing when individuals became obese and whether they began the study with obese readings.
"It's not that obese or non-obese people simply find other similar people to hang out with," Christakis said. "Rather, there is a direct, causal relationship."
They also ruled out the idea that an outside factor, and not the friendship, caused the fatness. If an environmental factor were affecting both individuals in a friendship, then it shouldn't matter whether individuals are mutual friends or just one individual labels the other as a friend.
The study, however, found that it does matter which way the friend arrow points: If subjects named an obese person as a friend, they tended to be affected by that person's obesity.
But when the person on the receiving end did not label the first person as a friend, there was no "obesity contagion" effect in the other direction. The distinct variable here is who calls whom a "friend."
"The fact that it only has an effect when I think you're my friend is very strongly suggestive to me," Watts said. "That's about as good as you can do in terms of identifying a causal relationship."
Perhaps friends just spend a lot of time together and so would eat similar foods and engage in the same physical activities. But they found the results held no matter the geographic proximity of friends.
"So friends that are thousands of miles away have just as large an impact on you as friends who are right next door," Fowler told LiveScience.
The scientists suggest the findings can be explained if friends are influencing one another's norms for body weight.
"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size," Christakis said. "People come to think that it is OK to be bigger since those around them are bigger, and this sensibility spreads."
