A few years ago, Nicholas Christakis and James Fowler made a striking discovery about obesity: it spreads from person to person, much like a contagious virus. They were able to demonstrate this by mining the data sets of the Framingham Heart Study (FHS), a longitudinal survey that has revealed many of the risk factors underlying cardiovascular disease. Because the FHS noted each participant's close friends, colleagues, and family members, Christakis and Fowler were able to recreate the social network of the town, to see how everyone was connected to everyone else.

And this is when they made their remarkable discovery about weight gain. According to the data, if one person became obese, the likelihood that his friend would follow suit increased by 57 percent. (This means that the network is far more predictive of obesity than the presence of genes associated with the condition.) If a sibling became obese, the chance that another sibling would become obese increased by 40%, while an obese spouse increased the likelihood that the other spouse would become obese increased by 37%.

The Christakis/Fowler work is an important reminder that Donne was right: No man is an island/entire of itself. Instead, we are all plugged into a vast network of social contacts and cultural norms. While we think ourselves as autonomous individuals, that autonomy is severely constrained by those around us.

But this longitudinal data - it's a bird's eye view of human life - still begs the question: How do other people influence us? Why does an obese friend make us so much more likely to gain weight? Why do the habits of others influence our own habits?

A brand new paper by researchers at the University of Colorado at Boulder helps answer these important questions. The scientists begin their paper with a compelling hypothetical:

Consider the following: Your friend Lucy, who is about 25 pounds overweight, e-mails you pictures from her recent vacation. After you look at Lucy’s pictures, the ofﬁce secretary comes by with a plate of cookies. Will exposure to someone who is overweight inﬂuence how many cookies you eat?

When asked this question, a majority of people insist that the picture of Lucy would reduce their consumption of cookies. (31 percent believed that Lucy would inspire them to abstain entirely from the sweet treat.) This is how we like to think ourselves: independent minded creatures, able to learn from the unflattering photographs of others.

Alas, our responsible self-image is entirely divorced from reality. The Colorado researchers demonstrated that, in several situations, the exact opposite occurred: When people were exposed to pictures of someone who was overweight, they ended up consuming far more calories.

In one of their experiments, researchers asked random strangers walking through a lobby at the University if they would take a quick survey. The surveys had photos of an overweight person, a person of normal weight or a lamp. After completing the survey, the researchers asked the subjects to help themselves from a bowl of candy. Those who were exposed to the picture of the overweight individual took, on average, 3o percent more candies than those exposed to the control pictures.

In a second study, subjects were invited to do a cookie taste test. Those who were first exposed to pictures of overweight individuals ate twice as many cookies as those were exposed to images of trees, fishbowls and non-overweight subjects. This effect held when participants said they had a goal to maintain a healthy weight. As the researchers write, "Exposure to a negative stereotype [seeing someone who is overweight] can lead to stereotype conducive behavior." Even when we are determined to maintain our diet, we are still subtly undermined by the choices and habits of everyone else.

This research builds on a 2010 paper by Northwestern psychologists that demonstrated that people anchored their own portion sizes to the portions around them. If we're surrounded by people eating a supersized Big Mac meal, then we're much more likely to do the same.

Taken together, this research begins to explain how obesity moves through a social network. It turns out that the habits and hungers of others shape our own, that we unconsciously regress to the dietary norms around us. Because we're not particularly good at noticing when we're sated and full - the stomach is a crude sensory organ - we rely on all sorts of external cues to tell us how much to eat. Many of these cues from other people, which is why our eating habits are so contagious.