Men initiate sex more than three times as often as women do in a long-term, heterosexual relationship. However, previous research shows that sex happens far more often whenever the woman takes the initiative, suggesting that it is the woman who thus sets the limits to a greater extent than men do.

Psychologists at NTNU have investigated what other factors play a role for the frequency of intercourse in couples in long-term relationships. Two factors are decisive in how often women take the initiative at all.

Attitudes to casual sex

Women's attitudes to casual sex play a major role, which may seem strange at first glance when talking about sex in long-term relationships, because we're not talking about extrapair affairs.

"This measure describes how much women distinguish between the sexual aspects of a relationship and its relational and emotional aspects," says Professor Leif Edward Ottesen Kennair at NTNU's Department of Psychology.

Women who tend to be more open to casual sexual relationships differentiate between positive, physical aspects of sex and relational and emotional aspects of a relationship to a greater extent. A quarrel about the dishes or who vacuumed last may therefore not be as crucial to whether the couple has intercourse.

Often, whether one has sex or not is a compromise between the parties, and women who differentiate more between sex and other aspects are probably more willing to compromise. Men are ready to have sex to a much greater extent, regardless of his attitudes.

But the woman's attitude to short-term sexual relationships, her sociosexual orientation, is not the only factor.

Got to have passion

"Passion in the relationship is of great importance for intercourse frequency," says postdoctoral fellow Trond Viggo Grøntvedt at the Department of Psychology, who is the first author of a newly published article in Evolutionary Behavioral Sciences.

The psychologists at NTNU considered several factors in their study, such as how happy people are in their relationship, how committed they feel to their partner, how intimate they are, how much they trust each other and the love between them.

All of these factors certainly have their good sides. But too simple to say that couples have more sex just because they love and trust each other. Only the passion in the relationship can predict the frequency of sex. "Passion is actually the only one of these factors that matters. We didn't find any association between any of the other aspects and how often people have sex in couple relationships," says Grøntvedt.

The study included 92 couples aged 19 to 30. Relationships varied in length from one month to nine years, with an average of just under two years. The couples had sex two to three times a week on average.

Desire for others reduces passion

The longer the relationship has lasted, the less often the couples had sex. And one other factor in particular reduces the frequency. "Love is a commitment mechanism, and there is less passion and desire in a relationship if a partner is more interested in others," says Kennair.

"Strong sexual fantasies about people other than the partner don't mix well with passion in the relationship," says Associate Professor Mons Bendixen, also at the Department of Psychology.

Only women's attitudes decisive

"The most remarkable finding is perhaps that it's only the woman's attitudes to casual sex that affect the frequency of sexual intercourse," says Kennair.

However, the findings may not apply to all cultures, Bendixen notes. They primarily apply to societies with more gender equality and female sexual control.