Who's at fault when sexual desire disappears in a relationship? New Kentucky study explains.

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In popular culture, it's always the woman who says: "Not tonight, dear, I have a headache."

But when it comes to sexual desire in real long-term relationships, men and women have more similarities than differences, according to a new study by University of Kentucky researchers in the Journal of Sex Research.

"The assumption that men have higher sexual desire than women overall is simply not consistently supported by the data in the context of relationships," write Kristen Mark and Julie Lasslo in a new article that looked at 64 different studies on sexual desire written since the 1970s.

In fact, the authors say, desire ebbs and flows between both partners for a variety of reasons, many of them more related to societal expectations. Men who feel insecure about a relationship might be less ready to initiate sex, while desire in both partners can be affected by factors like childbirth or illness.

"We've got these gendered expectations around, they're doing a disservice to both men and women," said Mark, who directs UK's Sexual Health Promotion Lab in the UK College of Education's Department of Kinesiology and Health Promotion. "When men have lower sexual desire, they're not meeting society's expectation to always be ready for sex. And then for women, it's also problematic, because when women have more desire than their partners, it can feel foreign to them for a variety of reasons."

The same societal tropes are also problematic for same-sex relationships, Mark said.

"It's not useful to make claims about large groups of people," she said. "It's better to look at individual differences and acknowledge every relationship will have ups and downs, regardless of orientation."

Mark said she was also interested in their findings about sexual expectations for long-term relationships.

"The idea that for you to expect your sexual desire to be the same forever — you set yourself up for disappointment and that can impact relationship satisfaction," she said. "One should expect their sexual desire will change throughout a relationship, that doesn't mean the relationship is doomed. It just indicates that sexual desire is quite complex."