Fertility giveaway in female voices

The way a woman sounds varies with her menstrual cycle and is an unconscious indicator of fertility, as Nathan Pipitone, from Adam State University, Colorado explains to Chris Smith... 

Nate - A lot of our main attractions seem to come from the visual domain. So, we're looking at features of for example, characteristic features of males - we're looking at stature, we're looking at muscularity, we're looking at confidence, types of behaviours, and so on and so forth. But we're not all visual creatures. And so, we actually use all of our senses when it comes to in person mate assessments so not only the visual domain, but in the auditory domain, which is the focus of my work, and even the olfactory domain, how somebody smelled, and even surprisingly, the gustatory domain. A colleague of mine has shown that kissing behaviour conveyed a lot of our underlying biological information to one another.

Chris - We learned on the programme last week that a kiss also conveys something in the region of 80 million different microbes per 5-second intimate encounter.

Nate - Exactly. So, some of those microbes are conveying good informations, some of those microbes are conveying some bad informations. So, lots of biological transmission, if you will, in a kiss.

Chris - And all these gets integrated in your brain, forming an opinion of that person and whether or not, you like them. Tell me more about the sound side of things then.

Nate - Some of my work has shown that women that speak higher in pitch, and we've done this in a natural fertility population, they actually produce more children and grandchildren. And so, fundamental frequencies seems to be one of the most important vocal acoustics that allows us to assess good mates from bad mates.

Chris - Is that biologically plausible? Why should a woman who is more fecund, more fertile, have a higher pitch to her voice than one who's less likely to bear offspring? Is it just because someone who's less likely to bear offspring with a low pitch voice is because they look more masculine and therefore, less attractive so they get actually fewer partners?

Nate - In terms of females, we think it's due to how oestrogen not only suits the female voice, but also, is related to female fertility. And so, women speaking on a higher pitch voice, we think that this is an indicator of trait oestrogen levels. And so, not only is the oestrogen a very important component in reproduction, but it's also shaping the larynx and it's shaping how women are speaking as well. This is why we think there's this systematic correlation between pitch and fertility.

Chris - Over the course of a month, a woman's menstrual cycle means that her hormone levels do wax and wane, oestrogen foremost among them, peaking in the middle of the cycle around day 14 when you are - on average - most likely to conceive if you have sex then. So, does this mean that a woman's voice changes across the month then and perhaps the signal about when you're most fertile coincides without high oestrogen peak?

Nate - This is precisely what we think is going on. And so, when we look at how a woman produces speech across the menstrual cycle, we've been able to document a systematic change in the level of attractiveness attributed to women that are naturally cycling. If we record the voice from the same female at different points of the menstrual cycle and then playback those voices to independent raters that haven't seen them, their voices recorded at high fertility are rated as more attractive compared to voices recorded at low fertility. It's an overall subtle effect with the very systematic shift that we've been able to pick upon.

Chris - Can you give us some samples of what it sounds like in the - let's say, low fertile state versus the high fertile state?

Nate - So in this sample, you'll hear a voice recording of women counting from 1 to 5 and this is the low fertility recording. 

Lady - 1, 2, 3, 4, 5...

In another recording, you'll hear the same woman counting from 1 to5. But the only difference is, that this was recorded at the time of high fertility.

Lady - 1, 2, 3, 4, 5...

Chris - Now, they do actually sound a bit different and there's no way that the woman just talks like that on one occasion and the other way on another occasion. This is a consistent finding that she sounds more high pitched on the more fertile situation.

Nate - What's interesting is between individual's pitch makes a difference. And so, as one woman speaks, we'll compare that recording in terms of attraction as to another woman speaking, pitch seems to make a difference. But within the same individual, one study has found that pitch does systematically change. But there's been several other findings that haven't been able to replicate that. And so, we can't just boil down what's attractiveness this across the menstrual cycle to voice pitch. There's a host of acoustics that are probably changing in systematic ways. What's interesting is that we don't need the acoustics to actually find the effect. The human brain almost instantaneously picks up on this slight attractiveness difference. So, we still have a lot of work to do in this area in terms of the approximate vocal acoustics actually affecting voice attractiveness.

Chris - And a group of people rated these voices according to their attractiveness without actually seeing the person speaking. So, they weren't being biased by what that individual looked like, purely by what they could hear?

Nate - Exactly. It's just by the sole sound of their voice. And so, our work has shown that just by speaking to other individuals, the sound of your voice conveys a host of biologically relevant information that correlates with attractiveness indices in the visual domain. And so, just by using the sound of somebody's voice, you can pick up a lot of information related to whether you're going to be attracted to that person.

Chris - Now, what about individuals who are using oral contraceptive pills, for example, which effectively fool your body into believing that it's permanently pregnant, the result being that you just don't ovulate and you don't have this big flux in hormones with a surge in the middle of the cycle. Do those individuals not show this behaviour then, one would predict that they wouldn't?

Nate - Precisely. And so, with all of our research, we always conduct a control group of women, we record their voices at different times of their menstrual cycle but then they are using hormonal contraceptive. And so, we look at voice attractiveness differences at "high fertility times" in these women compared to low fertility times and we haven't been able to find anything with women using hormonal contraceptives. So, using hormonal birth controls seems to truncate this attractiveness response across the menstrual cycle.

Chris - Does that in turn then mean that there's the possibility that if someone mates and dates while they're taking the oral contraceptive pill that subsequently, where they to stop the pill, they could be in for a disappointment?

Nate - Exactly. That's a really interesting implication. And so, we'll work as soon, looking at women that use hormonal contraceptives, they seem to prefer less masculine types of characteristics with men. So, they prefer more feminine male traits and these are in the behavioural domain and in the physical domain. So, not as much muscle mass, not as a robe or impulsive. These are characteristics that naturally-cycling females tend to like more than women using hormonal contraceptives. So, this is interesting for monogamous pair bonds. If women are using a hormonal birth control and they found a mate and then they want to start a family some day and they decide to go off of the pill or whatever they're using, they might find that their mate isn't what they thought they were. So, this is really interesting to see how hormonal birth control is affecting relationships throughout the world.