Contact Us

Staff

BU-Stockholm University study
Much less than six degrees can separate sexual partners

By Brian Fitzgerald

With surprising and somewhat frightening results, the concept behind "six degrees of separation" has been applied to sexual relationships in a study by Luis Amaral, a research associate at the GRS Center for Polymer Studies, and colleagues at Stockholm University. The "six degrees" theory is that everyone in the world is just six acquaintances away from everyone else.

Luis Amaral, a research associate at the GRS Center for Polymer Studies.

The researchers, studying data from a sex survey of 2,810 Swedes, checked how many sexual partners respondents had had in one year. They then estimated the number of sexual encounters needed to link two people. "We found that it could be much less than six," says Amaral. "Any two people might be only three connections apart."

The study, published in the June 11, 2001, issue of the journal Nature, received news coverage around the world, eliciting such headlines as "Sex Degrees of Separation" (BBC News Online) and "Six Degrees of Copulation" (New Scientist). However, aside from the obvious shock appeal for the media -- one respondent reported more than 800 sex partners in his life -- the research has serious epidemiological implications in a world that has 36 million people infected with HIV or AIDS, a figure that is 50 percent larger than the World Health Organization had predicted 10 years ago.

Entitled "The Web of Human Sexual Contacts," the article in Nature, cowritten by Amaral, CAS Physics Professor H. Eugene Stanley, Center for Polymer Studies director, and Stockholm University Sociology Professors Fredrik Liljeros, Christofer Edling, and Yvonne Aberg, concludes that sexual health campaigns could be far more effective if targeted toward the most promiscuous in a community -- the node, or center, in a web of sexual contact.

"We expected to find a small percentage of promiscuous people," says Amaral, "but it's not as small as you'd expect." The mean is approximately 15 sex partners in a lifetime for men, and 7 for women. To investigate the connectivity in the network of sexual contacts, the researchers analyzed the number of sexual partners over a relatively short time period: the 12 months before the survey.

"The study helps explain the spread of the AIDS epidemic," says Amaral. "At first, people thought that AIDS would be confined to the so-called risk groups, and then it would die out, and the general population would be safe. Many countries operated under that assumption. But then it spread through significant populations in these countries."

How does a researcher with a Ph.D. in physics end up coauthoring a study on sexual relationships? Amaral is interested in the structure of complex networks. And the Center for Polymer Studies, a scientific visualization research center of the CAS physics and mathematics departments, is devoted to interdisciplinary research in aspects of polymer, random, and fractal systems. "We use physical methods to study social and economic data," says Amaral.

In terms of social science, the six degrees of separation concept traces its roots to a 1967 study by psychologist Stanley Milgram, then at Harvard. The phenomenon entered the public consciousness in 1990 with the debut of the John Guare play Six Degrees of Separation, which was made into a movie in 1993. In 1994, three Albright College fraternity brothers invented a trivia game using this concept and trying to link any actor, living or dead, to Kevin Bacon within six films. In 1997, the mathematical basis for this phenomenon was proven by a Cornell University engineering professor, Steve Strogatz, and his graduate student, Duncan Watts, who found that "bridges" -- people who unite disparate social groups -- were the key to the system, which they dubbed the "Small World" phenomenon. Their research was published in the June 4, 1998, issue of Nature.

Amaral is currently involved in a similar study on sexual behavior in the United States, based on a 1995 study of 3,432 people by George Herbert Mead, a University of Chicago sociology professor. "The study in Sweden was inspired by Mead's study," says Amaral. The New York Times Book Review called Mead's research "the most important study of American sexual behavior since the Kinsey reports of 1948 and 1953."

"The Web of Human Sexual Contacts" comes on the heels of another American study concluding that one in five teenagers has an undiagnosed sexually transmitted disease. Amaral says that an extensive survey of the types of people likely to have large numbers of partners is needed so family physicians can identify and educate them before they become promiscuous.
When it comes to sex, it's truly a small world after all. "People who think that they are in no way connected to those with sexually transmitted diseases are very wrong," says Amaral.

7 December 2001
Boston University
Office of University Relations