While it may come as no surprise that genes may help explain why some people have many friends and others have few, the researchers said, their findings go just a little farther than that.

"Some of the things we find are frankly bizarre," said Nicholas Christakis of Harvard University in Massachusetts, who helped conduct the study.

"We find that how interconnected your friends are depends on your genes. Some people have four friends who know each other and some people have four friends who don't know each other. Whether Dick and Harry know each other depends on Tom's genes," Christakis said in a telephone interview.

Christakis and colleague James Fowler of the University of California San Diego are best known for their studies that show obesity, smoking and happiness spread in networks.

For this study, they and Christopher Dawes of UCSD used national data that compared more than 1,000 identical and fraternal twins. Because twins share an environment, these studies are good for showing the impact that genes have on various things, because identical twins share all their genes while fraternal twine share just half.

"We found there appears to be a genetic tendency to introduce your friends to each other," Christakis said.

There could be good, evolutionary reasons for this. People in the middle of a social network could be privy to useful gossip, such as the location of food or good investment choices.

But they would also be at risk of catching germs from all sides – in which case the advantage would lie in more cautious social behaviour, they wrote in the Proceedings of the National Academy of Sciences.