The 237 reasons why women have sex

Why do women have sex? More than a thousand interviews worldwide by a US research team unearthed 237 different reasons.

Reasons include having sex to impress their friends, to relieve period pain, punish a cheating partner, give someone they hate a sexually transmitted disease, love, to "get closer to God" and to keep a distracted partner interested.

(Read more: RALPH sex survey results)

The study was conducted by clinical psychologist Cindy Meston and David Buss, an evolutionary psychologist. Their findings have been turned into a book soon to be released in Australia, Why Women Have Sex.

Meston and Buss interviewed 1,006 women from all over the world about their sexual motivation, and identified 237 different reasons why women will have sex, "that covered an astonishing variety of psychological nuance."

They write: "Some women have sex to feel powerful, others to debase themselves. Some want to impress their friends, others want to harm their enemies."

Some women reported having sex to "fill a void" or gain love that they did not feel when they were growing up.

Other women had sex to raise self-esteem, finding it a balm after a bad breakup. Said one 19 year-old respondent: "it helps me feel better about myself, particularly if the person left me for someone else."

Some women had sex to get rid of migraines, as a stress reliever, to beat depression or to burn calories.

Others had sex because they were raped or dominated, while some woman loved the dominance that came with initiating sex and "demonstrating power" in the bedroom.

The researchers found that there was a strong connection between sex and love. "Most women seek some kind of emotional connection or emotional involvement with a man before consenting to sex. From an evolutionary perspective, this is emotional wisdom women have inherited from successful maternal ancestors," write the pair.

They say a man's emotional involvement provides powerful signals that he'll stick around for "richer or poorer."

The data was collected in online surveys and in lab tests. Ninety-three percent of the women surveyed were heterosexual, while the age range was between 18 and 86.

First contact

One of the tests assessing initial attraction proved that eye contact was the first step to the bedroom.

The study found proximity was an important factor of sexual attraction — but not necessarily too much contact. One study they conducted found that a brief series of face-to-face contacts of 35 seconds without even talking to the person increased positive responses.

The study found the effect of mutual eye gazing was important and "proved powerful."

"Many reported that deep eye contact with an opposite sex stranger created feelings of intense love," according to the book.

So powerful was eye-contact that participants reported being very attracted to their study partners — and two participants who were strangers even got married.

The study also said an element of mystery was a turn-on for women. Their research found that too much familiarity can backfire — and "positive traits can become annoying ... some men who were once funny and fun become 'embarrassing in public' and [some men who were] 'successful and focussed' become 'workaholic'."

One survey respondent wrote: "Mystery stokes attraction — familiarity can quash it."

Smells like business time

The study found one of the strongest ingredients of sex appeal was "women's acute sense of smell."

Researchers at Brown University back this up in a separate study, finding that women rate how someone smells as the most important sense in choosing a lover — with sight coming second.

The Meston/Buss study also found that a person's mood at the time of the initial encounter is an important factor in determining attraction as "positive feelings lead to positive evaluation."

They say anyone present when negative feelings are aroused tends to be disliked as a consequence — hence many women's wish to mate with someone who has a good sense of humour.

They also found women like men that have been "pre-approved" by other women. They respond better to pictures of men standing surrounded by other women than men surrounded by other men, or a man standing alone.

The more attractive the other women in the photos were, the sexier the women found the men.

Turn offs

The largest turnoff for women was poor hygiene. Interestingly, the research found changes in body shape such as weight gain can elicit a complex response.

Sometimes, if over time one partner gains weight, they feel less competitive in the sexual market which means they are more likely to stay with their current partner — and thus be more secure.

The study found the sex drives of women also decrease in tandem with the length of their relationships – so the longer a woman was with her partner the less likely they were to have sex with each other.