Now there’s even more scientific evidence for what most of us already know:  our friends can make us fat, happy, and sad.

 that good behaviors — like quitting smoking or staying slender or being happy — pass from friend to friend almost as if they were contagious viruses. The Framingham participants, the data suggested, influenced one another’s health just by socializing. And the same was true of bad behaviors — clusters of friends appeared to “infect” each other with obesity, unhappiness and smoking. Staying healthy isn’t just a matter of your genes and your diet, it seems. Good health is also a product, in part, of your sheer proximity to other healthy people. By keeping in close, regular contact with other healthy friends for decades, Eileen and Joseph had quite possibly kept themselves alive and thriving. 

When a Framingham resident became obese, his or her friends were 57 percent more likely to become obese, too. Even more astonishing to Christakis and Fowler was the fact that the effect didn’t stop there. In fact, it appeared to skip links. A Framingham resident was roughly 20 percent more likely to become obese if the friend of a friend became obese — even if the connecting friend didn’t put on a single pound. Indeed, a person’s risk of obesity went up about 10 percent even if a friend of a friend of a friend gained weight.

“People are connected, and so their health is connected,” Christakis and Fowler concluded when they summarized their findings in a July 2007 article in The New England Journal of Medicine, the first time the prestigious journal published a study of how social networks affect health. Or as Christakis and Fowler put it in “Connected,” their coming book on their findings: “You may not know him personally, but your friend’s husband’s co-worker can make you fat. And your sister’s friend’s boyfriend can make you thin.”

The researchers also found that happiness is contagious – more contagious than unhappiness.  “According to their statistical analysis, each additional happy friend boosts your good cheer by 9 percent, while each additional unhappy friend drags you down by only 7 percent. “

Christakis and Fowler began to wonder if a person’s connectedness is to some degree fated from birth — a product, at least in part, of DNA. To test the idea, they conducted a study of twins. Using the Add Health school data, they located more than 500 sets of twins and analyzed where they were located in their friendship clusters. Employing statistical techniques traditionally used to parse out how much of twins’ lifestyles are attributable to their genes as opposed to their environment, they found that almost half — 46 percent — of the difference between two twins’ levels of connectedness could be explained by DNA.

46 percent does not appear to be statistically significant.  As astrologers know, the ability of individuals to connect is reflected in their astrological charts.  Twins can be very different because although they have the same charts, the conflicts within the chart are often split between the twins resulting in very different personalities.