Women are more than twice as likely as men to lack interest in sex when living with a partner, a study of British sexual attitudes suggests.
It found that while men and women lost passion with age, women were often left cold by longer relationships.
Overall, poor health and a lack of emotional closeness affected both men's and women's desire for sex.
The findings are based on the experiences of nearly 5,000 men and 6,700 women, published in BMJ Open.
The UK researchers said problems of sexual desire should be treated by looking at the whole person, rather than simply resorting to drugs.
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Relate sex therapist Ammanda Major said losing interest in sex wasn't necessarily abnormal, and there were many different reasons why men's and women's needs changed.
"For some, it is a natural and normal place to be, but for others it causes pain and misery," she said.
In total, 15% of men and 34% of women surveyed said they had lost interest in sex for three months or more in the previous year.
For men, this lack of interest was highest at the ages of 35-44 while for women it peaked between 55 and 64.
But the researchers, from the University of Southampton and University College London, said there was no evidence that the menopause was a factor for women.
However, they did find that having young children at home was a particular turn-off for women.
Poor physical and mental health, poor communication and a lack of emotional connection during sex were the main reasons why men and women lost interest.
In the National Survey of Sexual Attitudes and Lifestyles in Britain, those who found it "always easy to talk about sex" with their partner were less likely to say they lacked interest.
However, those whose partner had had sexual difficulties, and those who were less happy in their relationship, were more likely to say they had lost interest in sex at some stage, the researchers said.
Among women, the study found that "not sharing the same level of sexual interest with a partner, and not sharing the same sexual likes and dislikes" were also a factor in loss of interest in sex.Image copyright Getty Images Image caption Is it him or me?
Cynthia Graham, professor of sexual and reproductive health at the University of Southampton, said the findings increased understanding of what lay behind men and women's lack of interest in sex and how to treat it.
"This highlights the need to assess and - if necessary - treat sexual desire problems in a holistic and relationship-specific, as well as gender-specific way."
She added that this was a problem that could not be fixed by a pill alone.
"It is important to look beyond anti-depressives," Prof Graham said.
The US Food and Drug Administration (FDA) recently approved the first-ever drug aimed at boosting female libido, called flibanserin.
Ammanda Major said: "Sex is a very personal thing, and talking about it can be embarrassing. But talking is often the best thing you can do to improve your sex life."