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changes across the menstrual cycle are known to produce some unique effects. Past research, for example, has found that women smell and walk differently during periods of high fertility. Even their faces change; women are judged to be more attractive when their chances of conception are at their highest.

While these effects may sound like the stuff of science fiction, there is a perfectly logical (and very Darwinian) explanation for it: It is another way for women to "signal" their reproductive status to potential suitors.

And, while past research has shown that women's voices are perceived to be more alluring during high fertility phases of the menstrual cycle, it was unknown exactly how women's voices changed to signal their reproductive status.

Until now. New research published in the journal examined the acoustics of female voices during periods of high and low fertility to identify the precise changes in vocal characteristics (i.e., pitch, loudness, and frequency) that were producing the effects.

To examine this, researchers at James Madison University and Florida Gulf Coast University recruited 19 naturally cycling women and 18 women on hormonal contraceptives (ages 17–30) to participate in a short experiment. In this experiment, the researchers asked women to record their voices four times over the course of a month. They were also asked to report the date of onset of their most recent menstruation.

Armed with this information, the researchers divided the recordings into four distinct groups: naturally cycling women during periods of high fertility, naturally cycling women during periods of low fertility, women on contraceptives during periods of high fertility, and women on contraceptives during periods of low fertility. Then, using a phonetics software program, the researchers examined specific vocal properties of the recordings to see what differences emerged between the groups.

Interestingly, they found that a voice property known as vocal "shimmer" was significantly lower among the naturally cycling women during periods of high fertility. Low shimmer, they write, "represents less variability in amplitude, ostensibly producing a steadier sounding voice." They also noticed that vocal "jitter" was directionally lower among the naturally cycling, high fertility group.

Together, lower vocal shimmer and jitter produce a clearer, healthier, and more mellifluous voice. Conversely, the researchers state, "Increases in both shimmer and jitter produce a sound that is perceived as rough or hoarse... . Increases in these same acoustics have also been used as a criterion for diagnosing dysphonia and other voice pathologies."

Another point is worthy of note. The researchers found no differences among the women using contraceptives when comparing their voices during periods of high fertility to periods of low fertility. It is likely that the hormones responsible for the producing vocal changes across the menstrual cycle are essentially "shut off" by hormonal contraceptives. Who knew that birth control could affect the quality of your voice?