The web of human sexual contacts: Promiscuous individuals are the vulnerable nodes to target in safe-sex camps

1 , social networks tend to be subjective to some extent 2,3 because the perception of what constitutes a social link may differ between individuals. One unambiguous type of connection, however, is sexual contact, and here we analyse the sexual behaviour of a random sample of individuals 4 to reveal the mathematical features of a sexual-contact network. We find that the cumulative distribution of the number of different sexual partners in one year decays as a scale-free power law that has a similar exponent for males and females. The scale-free nature of the web of human sexual contacts indicates that strategic safe-sex campaigns are likely to be the most efficient way to prevent the spread of sexually transmitted diseases. Many real-world networks 1 typify the 'small-world' phenomenon 5 , so called because of the surprisingly small average path lengths between nodes 6,7 in the presence of a large degree of clustering 3,6 (Fig. 1). Small-world networks are classed as single-scale, broad-scale or scale-free, depending on their connectivity distribution , P(k), where k is the number of links connected to a node 8. Scale-free networks, which are characterized by a power-law decay of the cumulative distribution P(k)Ϸk ǁȊ , may be formed as a result of preferential attachment of new links between highly connected nodes 9,10. We analysed the data gathered in a 1996 Swedish survey of sexual behaviour 4. The survey involved a random sample of 4,781 Swedes (aged 18–74 years) and used struc-tured personal interviews and questionnaires. The response rate was 59%, which corresponds to 2,810 respondents. Two independent analyses of non-response error revealed that elderly people, particularly women, are under-represented in the sample ; apart from this skew, the sample is representative in all demographic dimensions. Connections in the network of sexual contacts appear and disappear as sexual relations are initiated and terminated. To investigate the connectivity of this dynamic network, in which links may be short-lived, we first analysed the number, k, of sex partners over a relatively short time period — the 12 months before the survey. Figure 2a shows the cumulative distribution, P(k), for female and male respondents. The data closely follow a straight line in a double-logarithmic plot, which is consistent with a power-law dependence. Males report a larger number of sexual partners than females 11 , but both show the same scaling properties. These results contrast with the exponential or gaussian distributions — …