The eyes are often said to be the window to someone's soul, but the nose could be a backdoor to their bedroom. Experiments have found heterosexual women can actually smell which suitors are available and which are taken.

In recent years, the science of human scent has been sniffing up a storm in the lab, and recent results suggest that people who like to take deep whiffs of another's natural fragrance are likely to be more sexually motivated overall.

Straight men also seem to be more attracted to a woman's scent when their crush is at the most fertile point in her menstrual cycle, or when a woman is sexually aroused. Exposure to these pheromones can even trigger men to drink more on a night out.

Conversely, when women are ovulating, studies have found they are more attracted to masculine-looking males. Experiments also suggest that men's testosterone levels can subtly fluctuate depending on whether men they're single or in a committed relationship.

It's not yet clear if those hormonal changes can directly alter a person's appearance or their scent, but initial experiments suggest they might.

A 2019 Australian study asked 82 heterosexual women between the ages of 18 and 35 to rate the body odors and faces of heterosexual men. Half of the women were single while the other half were partnered.

Each female scored 3 single men and 3 partnered men randomly selected out of a pool of 89 anonymous participants. Researchers gave the men a t-shirt to sweat into for 24 hours, and the men provided a digital passport-style photo of themselves for the experiment.

The researchers snipped the underarms out of each t-shirt, which was stored in a screw-top bottle for the women to take a big whiff.

When smelling the anonymous bottled male odors, female participants were asked questions like "How much do you like/dislike this smell?" and "How sexy does this odor smell?".

The faces of these scents were then presented randomly to women. When looking at portraits, female participants rated the men on their attractiveness, sexiness, intelligence, loyalty, kindness, trustworthiness, masculinity, and whether they looked like a good partner.

Ultimately, researchers found that single men's body odor smelled stronger to all women than the natural scent of partnered men.

The more likable the male scent, the more likely women were to rate their looks as favorable.

Interestingly, partnered women rated single men's faces as more masculine than partnered men's faces. Single women rated them equally. That might sound curious, but the authors note that past research suggests that coupled women who are ovulating find the appearance of single men more attractive than partnered men.

Menstrual cycles and testosterone levels were not tested in the 2019 experiments, but the authors say their findings are "congruent with previous research showing that single and partnered males can be differentiated based on their testosterone levels, that higher testosterone levels are associated with a stronger smelling [body odor], and that more intense [body odors] are rated more masculine smelling."

Sex hormones, odor, and smell seem to be closely intertwined, and a theory of social neuroendocrinology helps explain why.

A study in 2010, for instance, found that single males have higher testosterone levels than partnered males. Not only could this make them more competitive in the dating arena, but the natural scent of their high-testosterone bodies could also signal fitness, viability, and sexual availability to others in an inexplicit way.

"From an evolutionary perspective, it may be advantageous for women to be able to detect the chemosignals that connote coupledom and ultimately avoid courting partnered males (especially with offspring) due to the relatively reduced resources they can offer," the authors of the 2019 study write.

But there is another explanation, and it's far less appealing: It's been suggested that married men have better health and hygiene than single men.

What's more, some physical health conditions have been found to result in a detectable change in body odor. Diet can also change how you smell to a certain extent.

Perhaps it is not high testosterone that is being smelled on single men, but the lifestyle effects of their singledom.

Future experiments among larger cohorts are needed to clarify some of these specifics. In the 2019 study, for instance, male participants were not allowed to use perfume or body cleansing products when wearing their t-shirts.

In 2009, however, a study found that young male students who used antimicrobial spray or fragrance oil felt more confident and attractive.

Smell is arguably the most scientifically overlooked human sense, and yet our sexual and social behavior seems to be closely tied to our noses.

Part of the problem is that we are not wholly aware of the way that smell influences us. Smell signals actually bypass a part of our brain known as the thalamus, which plays an important role in attention and consciousness.

When you're picking up someone in a bar, there's every chance you're unknowingly picking up their scent, too.

The study was published in Frontiers in Psychology.