The long quest to identify human pheromones and the smell of attraction, aggression and fear

What if anxiety could be treated with a simple nasal spray, instead of time-intensive psychotherapy or SSRI medications laden with side effects?

That is the admitted “wild dream” of Israeli neurobiologist Noam Sobel, who is optimistic enough to believe current research into airborne molecules linked to fear and other emotions could someday lead to medications that sound far-fetched now. His lab at the Weizmann Institute of Science in Rehovot, Israel, is one of many around the world studying human pheromones, or what Sobel calls “social chemosignals.” 

You might be quick to give pheromones the credit for your marriage, or maybe the blame for your breakup. But the scientific reality is that while researchers have long known that insects, fish and even some mammals exchange chemical signals that can affect behavior, the existence of pheromones in humans is still unproven, even 50 years after the first prominent experiments claimed otherwise.

“It’s a very controversial field,” says Yin Wu, a neuroscientist at The Hong Kong Polytechnic University. 

Let’s break it down.

What is a pheromone?

The term comes from the world of insects, where it means a chemical produced and secreted by an individual organism that triggers a specific change in the fertility, development or behavior of other individuals nearby. 

There is strong evidence that insects secrete pheromones for all sorts of purposes: termites to spread an alarm, moths to flirt and caterpillars to leave a helpful trail, for example. Some mammals, mainly mice, are also being studied for their use of pheromones.

But for humans, the picture is much less clear. For one thing, there’s no consensus on what a human pheromone even is. 

Stuff that humans regularly secrete, like sweat and urine, can be detected by other people, who then react to it. But that doesn’t mean these secretions count as pheromones because they don’t trigger a specific, measurable and consistent change in the people around them — at least, as far as researchers can tell. 

How might pheromone signals be sent and received in humans? 

Insects secrete pheromones from specialized glands throughout their bodies. Humans don’t have the same anatomy, but we do send molecules into the environment via sweat, urine, saliva, tears and breast milk, which is similar to how mice and other mammals secrete their pheromones.

But it’s not enough for humans to just be able to secrete a compound — for it to be a pheromone, other individuals have to be able to detect it. In mammals known to use pheromones, such as mice, the detection is done by the vomeronasal organ, which is lined with receptor cells that can identify airborne liquids entering the nasal cavity from the environment.

Most experts in the field agree that adult humans do not have functional versions of that organ, but Sobel isn’t so sure. The assertion that humans lack functional vomeronasal organs is based on “weak, null results,” he says.

Without a vomeronasal organ, it’s not clear how pheromones could be detected by human bodies, though pigs have been shown to sense pheromones even when their vomeronasal organs are surgically blocked. Some experts have suggested the human systems that recognize “normal” smells could do the same for pheromones.

After they are detected, pheromones are thought to influence the limbic structures of the brain, which also control smell, emotion and memory. Using limbic components, pheromones can theoretically trigger behavioral responses, though it’s still not clear how that would happen in humans.

Have researchers been hunting for human pheromones for a while?

Yes, a long while. 

One of the first high profile papers to suggest humans might be sending and receiving pheromone signals was published in 1971. Psychologist Martha McClintock, then at Harvard University, suggested pheromones could be responsible for the synching of menstrual cycles in women living in the same dorm at Wellesley College. 

But her work was later criticized for statistical errors, including the exclusion of certain data, that seemingly changed the results to make cycles look in sync when they really were not. “It’s all hogwash,” says Richard Doty, director of the University of Pennsylvania’s Smell and Taste Center.

Further research could not find evidence that menstrual cycles sync, suggesting instead that people might mistakenly perceive synchrony because of cycle overlaps. Several large reviews of research on the topic conclude the same way: menstrual synchrony seems to be a myth.

In 1975, Michael Russell, a pharmacologist at University of California, San Francisco, found that people could usually identify a T-shirt wearer’s sex by the odor of the shirt. It seemed to Russell that some sort of sex-based pheromone made this identification possible.

After this conclusion was published, though, Doty and his colleagues showed that odors from men are generally stronger and less pleasant than odors from women, so participants were likely basing their assumptions about gender on these qualities. If so, that would mean that there was no proof of pheromone involvement in the T-shirt experiment.

Why are people so captivated by the idea of human pheromones? 

To many people, the idea that one person can influence another via chemical signals is too exciting to disbelieve, which is why references to pheromones regularly pop up in all corners of pop culture. 

For example, Douglas Adams’ The Hitchhiker’s Guide to the Galaxy franchise from the 1970s and ‘80s includes a character who can control people using pheromones. In a 1999 episode of NBC’s ER, a patient suggests a connection between a womanizing doctor and his pheromones. And a character in Steven Soderbergh’s 2007 movie Ocean’s Thirteen rubs pheromones on himself to try to seduce and distract a heist target — it works in the movie, unlike in real life. 

More recently, pheromone perfumes have been widely advertised on social media with a promise that the wearer will attract eager admirers. A search for “pheromone perfume” on Amazon.com yields products with tens of thousands of positive reviews, even though the science is not sound. The New York Times even wrote about the idea back in 1993.

What are the most promising candidates for a human pheromone?

In the 2010s, a steroid called androstadienone became a widely favored contender for a human pheromone. Male sweat contains relatively high concentrations of this steroid, leading researchers to wonder if it signals the presence of men, which is why its effect on women is often studied.

A 2013 study, for example, found that exposure to androstadienone puts women in a better mood and heightens their focus, which can help with sexual function. Four years later, though, another study found the steroid did not affect people’s perception of the opposite sex, countering the argument that it is a human pheromone. The question is still not resolved, even as researchers continue to look at androstadienone. 

Another candidate has emerged more recently — hexadecanal, a volatile organic compound released into the environment from human saliva, skin and feces, seems to have some surprising effects on behavior.

Based on a study in mice, Sobel’s group in Israel originally theorized that exposure to hexadecanal would decrease aggression in people. Instead, they found that while they were right about men, exposure to hexadecanal actually increased aggression in women. It was, Sobel says, an “utter surprise.” 

Sobel thinks that men and women respond differently to this potential pheromone for evolutionary reasons. Hexadecanal is released by babies, so it may make mothers more likely to protect them aggressively, while simultaneously keeping fathers from becoming predatory towards their own children, he says.

“What you have here is a complete story: a [chemical signal], its behavioral significance and its existence in human secretions,” Sobel says.

But Pennsylvania’s Doty remains unconvinced by recent human pheromone research, including the science surrounding hexadecanal. “The jury’s out on a lot of those studies,” he says, because most have not been replicated. 

After decades of inconsistent findings, could a breakthrough come soon?

Maybe not. Doty is pessimistic, citing the inherent complications of pheromone research. 

There are “too many uncontrolled variables that are hard to take into account,” he says. As a result, many pheromone studies happen “without proper control groups, without blinding, double blinding, all that sort of stuff.” High-quality studies are especially expensive. 

Some researchers, including Hong Kong’s Wu, have abandoned the field. “It’s difficult,” he says, explaining that he recently changed his focus within neuroscience.

But Sobel hasn’t lost his optimism. As he works to understand the biological mechanisms that humans might use to process pheromones, he is also searching for more pheromone candidates. 

One potential clue: research has shown that women’s emotional tears reduce aggression in men, but it’s not clear which molecules are responsible. Identifying these molecules would not only be a scientific breakthrough, but could also potentially be used down the line to control aggression through prescription sprays.

Similarly, researchers are looking for the compound in sweat that signals fear. The “smell of fear” has been the “flavor of the month for the last five years in this field,” Sobel says.

But there are thousands of different kinds of molecules in sweat, so finding the one, or ones, that signal fear could take many years. It is also unclear whether this pheromone would persist in the environment or break down quickly. If the latter, finding it could be even more difficult.

It’s a “needle in a haystack kind of search,” Sobel says, noting that his team has already spent several years and a lot of money looking for a fear pheromone in humans. If they find it, though, his vision of an anxiety-preventing nasal spray could one day be a reality.

No matter what happens, public interest in pheromones is unlikely to wane any time soon. “People want exciting things,” Doty says. “The more fantastic these [findings] are, the more attracted we are to them.”