Six degrees of sexual separation?
Mathematicians see patterns in everything and it seems human sexual relations are no exception.
Indeed, the social networks formed when humans have sex have a very clear mathematical structure, say Swedish and American researchers. They believe their findings could lead to improved designs for safe sex education campaigns.
Their analysis, based on a 1996 Swedish survey of sexual behaviour, suggests the mating game really does show evidence of a "small world" phenomenon. Because some people have an unusually large number of sex partners, most of us are only a few degrees of "separation" from each other.
Published in today's issue of the journal Nature, the investigation describes how the human sex web fits what mathematicians call a "scale-free network", formed by the "preferential attachment of new links between highly connected nodes". In other words, in sexual contact networks, "the rich get richer".
The authors suggest this is probably a consequence of our varying degrees of attractiveness, our increased skill in attracting new partners as the number of previous partners grows, and the motivation to have many new partners to sustain self image.
Since epidemics arise and propagate much faster in networks of this kind, the finding could have implications for health education campaigns. The "well-connected" individuals are the "vulnerable nodes" in the network, the researchers say. If disease strikes them, the consequences are likely to be more serious.
"Strategic targeting of safe-sex education campaigns to those individuals with a large number of partners may significantly reduce the propagation of sexually-transmitted diseases," the paper says.