Women have sex to relieve boredom, keep the peace - and even to ensure their husband puts the rubbish out. They're among the findings of a new book, Why Women Have Sex, which cites 200 reasons after discounting the obvious physical attraction.

The authors, US-based psychologists Cindy Meston and David Buss, interviewed 1000 women worldwide.

They conclude romance and passion rate low, with 84 per cent of women having sex to ensure a "quiet life" or as a bargaining tool for household chores.

New Zealand relationship experts say the research is enlightening, but many questions remain unanswered about why women have sex.

Wellington-based sexologist Dr Michelle Mars said romance is probably low on the list because passion can easily dissipate during a long-term relationship.

"You might have sex to show your partner you still love and care about them - because you do - but maybe you don't fancy them in the same way any more.

"The attraction can wear off. And when that happens I suppose the sex can become quite lame."

She said many of us fall into a routine of having bad sex out of habit. "I think if the book asked the question 'Why do women have good sex?' we would have got some very different answers."

Auckland sex therapist Nic Beets said someone using sex to make their husband do chores may have a more complex reason they were unlikely to talk about.

"For example, a woman who grew up with a family dealing with alcoholic problems might be generally quite neglected emotionally as a child.

"When they grow up and enter a relationship they might reassure themselves they are wanted by having sex - but if you asked that person why they are having sex, they probably won't give you that answer."

Beets, who runs the Couple Work clinic with his wife Verity Thom, said the book assumes people know why they do things.

"By and large, people are not self-aware enough to give proper answers to these sorts of questions, and it can be difficult making people open up."

Many women still find it difficult to feel entitled to their own pleasure, said Beets.

* Reasons for sex

- One wanted to "relieve boredom" and said "it was easier than fighting".

- Another said it was "the closest thing to God".

- Some cited the generosity of their boyfriends, explaining: "he bought me dinner" or "he spent a lot of money on me early on" .

- One in 10 admitted having "sex for presents".

- And one had pity sex: "I slept with a couple of them because I felt sorry for them."

* Source: Why Women Have Sex