Estrogen, not testosterone, plays key role in revving women's sex drives

A flood of estrogen during the most fertile period of a woman's cycle helps boost libido, while an increase in progesterone coincides with a drop in sexual deire, according to the self-reported sex drives of women in a UC Santa Barbara study.
Estrogen in women can boost libido during peak fertile days, while progesterone dulls sexual desire, a new study suggests.

A new study on female hormones and sexual desire finds that estrogen, not testosterone, can boost libido in a woman's natural cycle, while progesterone deadens it, they say. Not coincidentally, this increase in desire works in favor of a woman's fertility.

"We found two hormonal signals that had opposite effects on sexual motivation," said lead author James Roney of the University of California Santa Barbara. "Estrogen was having a positive effect, but with a two-day lag. Progesterone was having a persistent negative effect, both for current day, day before, and two days earlier."

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Roney and his team recruited undergraduate students and measured their hormonal levels against self reports of sexual desire. The researchers saw a "measurable increase" in progesterone levels at the same time the subjects reported drops in libido.

The team also discovered a lack of impact of testosterone on women's sexual motivation, which counters a common belief. "Doctors tend to believe that, though the evidence isn't that strong in humans," Roney said. "In the natural cycles, we weren't finding effects of testosterone."

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However, testosterone does have a positive effect on sexual desire during hormone replacement therapy, but Roney said that may be a pharmacological effect.

"Testosterone has those effects if you inject it externally in women who are menopausal, and there are a lot of reasons that might be the case," he said. "For example, testosterone can be converted to estrogen through a particular enzyme. If you inject menopausal women with testosterone, it might be acting as a device that's delivering estrogen to the target cells. So the fact that it works doesn't necessarily mean it's an important signal in the natural cycle."