Ever noticed that people tend to be around the same weight as spouses and friends? Well, it's not your imagination. A study just out in the New England Journal of Medicine reports that obesity, a major risk factor for type 2 diabetes, acts somewhat like a contagious disease - so when one person puts on weight, others around them follow suit. This, the researchers say, helps explain why Americans have gotten so fat in recent decades.

The study, a summary and discussion of which is featured in The New York Times, involved the analysis of a social network of 12,067 people over a period of thirty-two years (from 1971 to 2003). Researchers tracked not only the health and weight loss/gain of these people, but also who knew whom, who was friends with whom, and who was related to whom. Over time, it became clear that those whose friends became obese were much more likely to grow obese themselves. The likelihood, in fact, tripled in the case of close friends. Interestingly, friendship mattered more as a determining factor than did being related or being neighbors with someone.

It works like this: people tend to share the lifestyle habits of their family and friends. Folks who consider salads delightful and who enjoy frequent jogs around the park generally surround themselves with similarly-minded folks, and it shows in their trim physiques. Likewise, fast-food munchers who enjoy lots of TV tend to hang out with a similar crowd...and their thighs - not to mention waistlines, hearts, blood sugar levels etc. - suffer the consequences. Another factor, says researcher Nicholas Christakis, is the perception of the self in relation to others: "You change your idea of what is an acceptable body type by looking at the people around you."