Having multiple sexual partners is a significant predictor of women turning to drink and drugs in later life, a new study suggests.
The study, based on a world-renowned programme that has followed 1000 people born in Dunedin 40 years ago, is believed to be the world's first to show the pattern.
Women who averaged more than 2.5 sexual partners a year in the years leading up to each interview with the researchers were 10 times as likely as women who had only one or no sexual partners a year to be clinically dependent on alcohol or drugs at age 21. They were seven times as likely by age 26 and 17 times as likely by age 32, even after allowing for all other factors in their lives.
The relationship was weaker for men.
A researcher in another long-running study in Christchurch, Dr Joe Boden, said the Dunedin finding was "novel" because everyone until now had thought the drink/sex linkage ran only one way - from alcohol to sex.
"It's an argument about reciprocal causality where it creates a kind of feedback loop," he said.
One of the Dunedin authors, public health professor Charlotte Paul, said the study pointed to mental health risks for women in having many sexual partners, and the obvious physical risks of sexually transmitted diseases.
"At present we are engaged in some very proper discussion about individuals committing to each other in marriage for gay people, but we don't seem to be talking about this at all in terms of heterosexual activity."
The study found that men born in Dunedin in 1972-73 were still more likely than women to have multiple sexual partners between 18 and 31.
Similarly 27 per cent of men, but only 12 per cent of women, were dependent on alcohol or drugs at age 26. "Dependent" means their drugs or drinking seriously affected their work or social relationships.
But for men, the number of sexual partners made little difference to their chances of being drink- or drug-dependent.
Women who had fewer than about 15 partners in their early 20s were less likely than men to be substance-dependent. But above about 15 partners they were more likely than men to be substance-dependent, and that likelihood increased steeply the more partners they had.
This pattern was found after adjusting for other factors, including previous drug and alcohol problems.
The researchers said the link between the number of partners and later substance dependence might be due to the "shared context" of drinking and meeting people in bars, or to both behaviours being related to underlying risk-taking attitudes.
But they said the much stronger effect for women suggested that having multiple partners directly heightened the risks of later substance dependence.
"Men and women may have different motives for having sex. Women often say it is for love, commitment and emotional reasons and while men also share these motivations, they may be more likely to also participate in sex just for physical reasons," they said.
"Self 'medication' with substances may be one way of dealing with this interpersonal anxiety. But much more needs to be known about why the association is so strong for women."