By F. Perry Wilson, MD, MSCE

We live in a world of sight and sound. That is the human experience. But most other mammals live in a very different world, a world of smells. Evolution has atrophied our sense of smell to the point that, in social situations, we don’t really think about it unless an odor is particularly offensive.

But it turns out that, somewhere deep in the recesses of our brains, we still respond to smell cues from other humans. And, for men at least, certain smells from women can make us more calm and less hostile, and make them seem more attractive.

And now, thanks to some rigorous research, scientists may have identified exactly the chemical compounds that have this effect on the male of our species. What chemistry makes up the scent of a woman? Let’s find out.

The study we’re discussing today, is not the first to note that smells can have physical and psychological effects on people. But it may be the most detailed.

Researchers recruited 21 female volunteers to be scent donors. They used a special silicone material, placed under the armpits, to capture the molecules that, under ordinary circumstances, float through the air and into our nostrils and up to our olfactory bulbs.

They did this across the four phases of the female menstrual cycle to figure out if and how scent changes with hormone level changes.

Then they exposed 21 men to these samples and asked them to rate them all in terms of pleasantness. You can see the results here.

While all the smells were rated slightly less pleasant than a “no odor” control, the scents collected during ovulation were rated by men to be significantly more pleasant than those during other phases of the menstrual cycle.

“Pleasant” is a rather subjective term, of course. The researchers asked the men to describe the odors across a spectrum that would be familiar to any perfumer or sommelier. During ovulation (labeled "O" here), men described women’s scents as more citrus, more “grassy,” and more “fragrant,” while being less “vinegary,” “musty,” or “stinky.”

But what exactly is making those smells? This is where the study starts to get really interesting. The researchers put those silicone patches through a mass spectrometer to identify all the volatile compounds present. They then identified compounds that were more uniquely present during ovulation compared with other times during the cycle. In this case, there were three big hits.

These three chemicals are the likely candidates for those pleasant aromas that men reported from the first set of experiments: (E)-geranyl-acetone, tetradecanoic acid, and (Z)9-hexadecanoic acid.

These are really interesting compounds. (E)-geranyl-acetone is formed by the breakdown of squalene, which is a substance on our skin. It is described as having a green or floral quality, similar to what the men in the first set of experiments described. You can find it in tomatoes, mint, citronella, passionfruit, and quite a few other places.

Tetradecanoic acid (myristic acid) has a waxy or creamy odor. In humans, you find this in breast milk, amniotic fluid, and saliva, in addition to the skin. Human babies will actually start a suckling reflex when exposed to tetradecanoic acid. In the natural world, the richest source of tetradecanoic acid I could find was in nutmeg butter, but despite being from the Nutmeg State, this is one foodstuff that has not yet graced my table. Still, the picture of exactly what makes the pleasant scent of a woman pleasant is becoming more clear. We have green grass, we have citrus, and something creamy that — perhaps — evokes breast milk.

But the third compound was a bit harder to understand. (Z)9-hexadecanoic acid is also known as palmitoleic acid. It is, apparently, odorless. It does, however, break down into (E)-2-nonenol which is a compound known to carry that “old age” smell that you sometimes hear about. I am unclear how this precursor to an old-age smelling compound plays into the bouquet we are describing, but it is there. Data don't lie.

Here’s where the study gets cool. With these three compounds isolated, the scientists created their own version, a mix of the three, perhaps the most scientifically based perfume in history.

Men were brought into a room and given a headset with a microphone. Unbeknownst to them, the microphone cover had been dosed with either nothing or one of three scents: the ovulation cocktail, the “baseline” armpit smell, or a combination of these two scents.

Then the men were asked to take some surveys. Nothing about smell this time; they were about mood. Unaware of what scent they were smelling, the men were not even aware they were supposed to be smelling something. The men exposed to the ovulation cocktail reported less feelings of hostility, increased “liveliness,” better concentration, and less boredom.

The researchers stop short of calling this “signaling via pheromone,” but it is hard not to conclude that a simple scent, in this case a purely synthetic one, can induce specific feelings in men.

The experiments didn’t stop there. They next had the men rate faces of various women. Shown a face, they were asked to rate them in terms of whether the person was beautiful, elegant, and intellectual, someone they want to spend time with, and someone they want to keep gazing at.

These four categories were all quite correlated, actually. It turns out that if a man thinks a face is beautiful, he also thinks he wants to spend time with that woman. In any case, for women rated very high on the attractiveness scale, the scent didn’t seem to make a difference; they always scored well.

But for women who, overall, were rated lower on the attractiveness scale, they scored better when the men doing the rating were exposed to that ovulation cocktail.

It’s sort of crazy to me to think of how our perceptions can be influenced by sensations we aren’t necessarily fully conscious of. And, of course, this research leads to some very interesting questions. Probably the first on people’s minds is: Does any perfume have these compounds in it?

And the answer is, yes, absolutely, but since perfume ingredients are not always listed, I have no idea which one. If you’re looking, aim for something that has notes of nutmeg, citrus, and tea, perhaps.

Of course, as a man, I wonder if this works the other way. We don’t have hormonal cycles with quite as profound physiologic ramifications as women do, but I do wonder if there are compounds in our natural body odor that might affect the feelings or thoughts of the fairer sex.

This study reminded me of an embarrassing period in high school when a relationship with my then girlfriend felt like it was on the skids and, desperate to hold things together, I changed my cologne. It turned out, as you might expect, that that was not the secret to long-lasting happiness. This research won’t unlock that secret either. But what we see here is the way in which we are starting to decode a new language, the language of smells, one that many thought humans were no more capable of speaking than dogs are of speaking English. But perhaps, like a dog recognizing the word “walk,” somewhere, in the recesses of our brains, we still understand the language of scent.

Sorry for being nosy.

F. Perry Wilson, MD, MSCE, is an associate professor of medicine and public health and director of Yale’s Clinical and Translational Research Accelerator.