Is the network of heterosexual contact in Japan scale free?

Elucidation of the structure of human sexual networks is not only an interesting topic in the area of social networks but also an important clue for understanding the spreading risk of sexually transmitted infections (STIs). Some previous studies have indicated that sexual networks are scale free, while others have suggested that they are not. We conducted a Web-based survey on sexual contact in Japan to collect data on cumulative (total) heterosexual partners and the number of recent (in the last three or previous three months) heterosexual partners. To determine whether the number of heterosexual contacts in Japan has a power-law tail, we used maximum likelihood fitting methods and Kolmogorov-Smirnov tests. For confirmation, we also used the Akaike information criterion (AIC) and the Bayesian information criterion (BIC). Our results indicate that the distributions of the number of sexual partners in Japan have power-law tails.