Pheromones are chemical signals that animals release to communicate with other members of their own species. Unlike hormones, which work inside the body, pheromones travel outside the body and trigger responses in other individuals, influencing everything from mating behavior to territorial boundaries to social bonding. The concept is well established in insect and animal biology, though its application to humans remains surprisingly complicated.
How Pheromones Work in Animals
Animals emit pheromones to advertise their presence and regulate social and reproductive interactions. These chemicals activate specialized receptors in the receiving animal, which then trigger changes in behavior, hormone levels, or both. The responses can be remarkably precise. Male silkworm moths, for example, have roughly 17,000 sensory cells on their antennae dedicated to detecting a single female sex pheromone called bombykol. A single molecule of this chemical is enough to activate those sensory neurons, allowing males to locate females from great distances.
Most mammals detect pheromones through the vomeronasal organ (VNO), a small sensory structure inside the nose that’s separate from the regular sense of smell. In snakes, the VNO helps with hunting and tracking prey. In rodents, it picks up signals related to mating readiness and aggression. Since the discovery of bombykol in the 1950s, hundreds of pheromones have been identified across species, particularly in moths and butterflies.
Four Types of Pheromones
Scientists classify mammalian pheromones into four categories based on what kind of response they produce:
- Releaser pheromones trigger an immediate behavioral change. A female moth releasing a sex attractant that draws males from miles away is a classic example.
- Primer pheromones cause slower, physiological shifts. These can alter hormone levels, delay or accelerate sexual maturation, influence testosterone in males, or modify reproductive cycles in females.
- Signaler pheromones convey identity information, helping animals recognize relatives, distinguish group members from outsiders, or identify an individual’s reproductive status.
- Modulator pheromones influence mood or emotional state. Interestingly, this category was first described through research on humans before being studied in other mammals.
Do Humans Have Pheromones?
This is where the science gets murky. Humans clearly respond to body odors in social and emotional contexts, but whether we produce true pheromones in the strict biological sense is unresolved. The problem starts with anatomy. While the vomeronasal organ is physically present in the vast majority of human adults, it appears to be a vestigial structure. It lacks the nerve cells and nerve fibers needed for sensory function, and humans don’t have the brain structure (the accessory olfactory bulb) that processes VNO signals in other mammals. The genes that code for the VNO’s receptor proteins in animals with functional pheromone detection have mutated and become nonfunctional in humans.
That said, the VNO isn’t the only possible route. Humans could process chemical signals through the regular olfactory system instead. Some researchers have recorded electrical responses when applying candidate human pheromones directly to the VNO region, and those same chemicals produced no response when placed on regular nasal tissue. But without functional neurons or a processing center in the brain, most scientists consider this insufficient evidence.
What the Research Shows in Humans
One compound that gets a lot of attention is androstadienone, found in male sweat. A 2023 study found that smelling androstadienone reduced both reactive and proactive aggression in men, while it increased reactive aggression in women. At a speed-dating event, women exposed to the compound rated men as more attractive than women who weren’t exposed. These findings suggest the chemical does something meaningful to human behavior, but the effects are subtle and context-dependent, not the powerful, automatic responses seen in insects.
Another popular claim involves menstrual synchrony, sometimes called the McClintock effect. First reported in 1971, the idea is that women living together gradually align their menstrual cycles through pheromone exposure. Some studies support this: one found a 54.8% shift toward synchrony among women sharing accommodations. But studies on cohabiting lesbian couples, women in Chinese dormitories, and natural fertility populations have failed to find significant synchrony. The cycle-altering pheromone, if it exists, has never been chemically identified or isolated. The phenomenon remains genuinely debated.
Body Odor, Attraction, and Immune Genes
One of the more intriguing lines of research involves the immune system’s influence on body odor. In mice, individuals prefer mates whose immune genes (specifically, the major histocompatibility complex, or MHC) differ from their own. This makes evolutionary sense: offspring with more diverse immune genes would be better equipped to fight a wider range of infections. An early human study found that naturally cycling women preferred the body odor of men with different immune profiles.
However, a comprehensive review combining data from genomic studies, odor preference experiments, and analyses of actual couples found no overall significant link between immune gene similarity and mate choice in humans. The average effect size was essentially zero. Some individual populations, particularly Swiss participants in early studies, showed a preference for immune-dissimilar odors, but this didn’t hold up across broader samples. Evidence for publication bias in this area further weakens the original claims.
Why the Term Gets Misused
The word “pheromone” gets thrown around loosely in marketing, especially for perfumes and colognes that claim to contain human pheromones capable of boosting attraction. These products typically contain compounds like androstadienone or estratetraenol, but the evidence for their effects on attraction is limited to controlled lab settings with inconsistent results. No chemical has been definitively identified as a human pheromone in the way bombykol is established as a silkworm moth pheromone.
Scientists who study chemical communication in humans often prefer the term “chemosignals” rather than “pheromones,” precisely because the strict definition requires a specific chemical that reliably triggers a specific response in members of the same species. Human responses to body chemistry are real but appear to be more complex, more variable, and more influenced by context than what the word “pheromone” technically describes.