Having more sex partners has been linked to higher risk of drug addiction and alcoholism. And the risk is especially great for women, according to new research. Researchers explored the relationship between addictions and risky sexual behavior in a report published in the journal Archives of Sexual Behavior. They followed virtually all of the 1037 children born between 1972 and 1973 in Dunedin, New Zealand, and asked about their sexual partners as well as alcohol and other drug use. Women who had more than two to three sex partners when they were 18 years to 20 years old were nearly 10 times more likely than those who had none or one sexual partner to develop a drug problem, primarily involving alcohol or marijuana, at age 21. Having more than two to three partners from age 21 to age 25 increased addiction risk at age 26 by a factor of 7. And at age 32, the risk was nearly 18 times greater for women who had more than two to three partners when they were aged 26 to 31 compared to those with one or no partners during that time. The risks for men were also increased, but not by as much. More than one sex partner from age 18 to age 20 nearly tripled the risk of a serious substance use disorder at 21— and having more than two to three partners quadrupled that risk for men of that age. While the connection isn’t surprising, the implications could be profound. The CDC reports [PDF] that 24% of women aged 20 to 24 have had two or more partners in the past year; the number for men was 29%. And women in particular may be likely to under-report this number. The results were adjusted to take into account the effect that mental illnesses may have on risky sexual behavior, including having multiple partners; the researchers also tried controlling for socioeconomic status but found that the strong relationship between multiple sex partners and drug addiction and alcoholism stayed the same. The authors also limited the study to heterosexual sex, defined as intercourse. In contrast to previous studies, the research did not find that having multiple partners increased risk for later anxiety or depression. The researchers, led by Sandhya Ramrakha of the University of Otago in New Zealand, speculate that there may be several explanations for the connection. Impulsivity can increase risky behavior of any type, and in some cases may be driving both the drug misuse as well as the sexual activity. “[P]eople who are impulsive may be more likely to engage in both activities and consequently [be] more likely to become substance dependent,” the authors write. In that case, says Howard Shaffer, director of the division on addiction at the Cambridge Health Alliance, which is part of Harvard Medical School, the study is “interesting, but not surprising.” Shaffer, who was not associated with the research, says, “Having sexual partners is risky and using drugs is risky. It might be that people who take one kind of risk also are willing to take the other. That the effect is stronger for women than men tends to support my interpretation because multiple sex partners is more taboo for women than men,” he says. “What drives both kinds of risk taking is the more interesting question.” It’s also possible, according to the authors of the paper, that “occasions of substance use are opportunities for sexual behavior because of its disinhibitory effects. Young people are likely to meet new sexual partners in situations where alcohol is served.” And many people drink for precisely that reason. But Ramrakha and her colleagues also suggest a final explanation that they find more “intriguing” than the others. Having many short-term relationships may itself be psychologically damaging. “[This] may be due to the impersonal nature of such relationships,” they write, “Or it might be that multiple failed relationships create anxiety about initiating new relationships. Self ‘medication’ with substances may be one way of dealing with this interpersonal anxiety.” Women, who are culturally expected to prefer monogamy, may be at higher risk than men if they do not do so. Any of the potential explanations raises important concerns about the common factors that might be driving sexual and addictive behaviors, and could open up new ways to identifying and protecting those who might be most vulnerable.