The same team that demonstrated obesity and smoking spread in networks has shown that the more happy people you know, the more likely you are yourself to be happy.
And getting connected to happy people improves a person's own happiness, they reported in the British Medical Journal.
"What we are dealing with is an emotional stampede," Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston, said in a telephone interview.
Christakis and James Fowler, a political scientist at the University of California, San Diego, have been using data from 4,700 children of volunteers in the Framingham Heart Study, a giant health study begun in Framingham, Massachusetts in 1948.
They have been analyzing a trove of facts from tracking sheets dating back to 1971, following births, marriages, death, and divorces. Volunteers also listed contact information for their closest friends, co-workers, and neighbors.