Carolyn Abraham
Medical Reporter

Sex begets small world, researchers find

'Six degrees of copulation' explains how a few promiscuous people spread disease

Just as six handshakes are thought to separate one person from every other in the world, researchers have found that even fewer than six rolls in the hay can connect an individual to the sexual partners of everyone else.

The so-called "Six Degrees of Separation" theory, developed more than 30 years ago and since popularized by the John Guare play, a Hollywood film and games on the Internet, suggests that anyone on the planet is connected to anyone else through no more than six others.

For Swedish and U.S. researchers, the theory has proven a handy tool in trying to unravel the "small world" web to figure out how sexually transmitted diseases can infect so many, so fast.

Their findings lend support to the warning public-health officials have long touted: that in sleeping with one partner, a person is essentially sleeping with everyone that partner ever had.

One man in the study, for example, reported that he had 800 sexual partners in his lifetime.

Based on the sexual behaviour of 2,810 Swedish men and women the researchers now estimate that all it takes is between three and seven sexual contacts to be linked to the sexual partners of anyone else.

"If you are thinking you are out of the risk group if you only have three or so partners, you are not," said Luis Amaral, the Boston University physicist who co-authored the report published today. (The science journal Nature actually promoted its paper with the tag line "Six degrees of copulation.")

According to Prof. Amaral, education campaigns to limit the spread of sexual disease should specifically target promiscuous people since the researchers' model shows that this worldwide sexual web is actually spun by a very small group with a whole lot of partners.

Public-health campaigns, he said, "are shooting into the air, instead of aiming at the ducks."

"These few people are having sex with enough new partners that they have the power to spread an epidemic," he said. The model helps explain how a virus such as HIV has killed 21 million in the two decades since it was identified.

But Ted Myers, a social epidemiologist and director of HIV-Social Behaviour and Epidemiological Studies at the University of Toronto, said there is still merit in safe-sex campaigns for the public at large.

"It is still worthwhile because you create healthier attitudes in the general population," Dr. Myers said, and the programs may be reaching partners of these very high-risk people.

As these men and women — one woman reported having 100 partners — are not in the traditional high-risk group of prostitutes, for example, Prof. Amaral recommended psychologists start a workup on sexual enthusiasts prone to collect partners like stamps.

While physicists are more often consumed with the micro-matter of the cosmos than the trends of the boudoir, "network theory" is a hot area in the field. One recent study, for example, examined how computer viruses propagate on e-mail and how fewer than a dozen clicks of the send button can spread it around the globe.

Prof. Amaral said researchers decided to refine the theory that everyone is "acquainted with one another" by six other people. The definition of "acquaintance" is murky, he said. But the definition of having sex is more clear.

"What is the definition of an acquaintance? Is it a handshake? How long really is the chain between me and someone in China; can I call them on the phone?"

He explained, the definition of sex is clearer, "if people had sex then they agree they had sex … unless of course you are Bill Clinton."

The 1996 Swedish study that the scientists analyzed found that most of the people, who were predominantly heterosexual and generally spanned the age brackets, reported having between one and three partners in their lifetimes. The average was 15 partners. But the high rankers by far were men.

Dr. Myers said a similar "network-theory" study from Manitoba in 1997 also found it was a small number of people who claimed extraordinary numbers of partners.

Asked whether he thinks 800 is an incredible number of partners for one person to have in a lifetime, Dr. Myers said, "You have to remember there are 365 days in a year … and some people may go through periods where they are highly sexually active."