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The Science of Smells

Do you sometimes notice that people naturally smell really good?

Maybe you’ve asked that question before, or maybe it just sounds like it makes sense. Levels of hygiene aside, sometimes people just straight up smell good to us. But what could be causing this to happen? What’s up with our noses? 

Science has managed to provide explanations for the many aspects of why we like certain people’s smells. Something you might have heard of before are pheromones— chemical signals released outside the body. These molecules are then picked up by another member of the same species (usually through smell), which triggers a change in behaviour. While this is often used in nature to attract mates, they actually have a wider variety of uses! For example, many insect and plant species use pheromones to send out alarm signals, and ants even use them to map out food locations.

The Science of Smells

If you think about it, pheromones are just a way to communicate—chemically.

However, for us humans the link between pheromones and attraction becomes much more complicated. In many other species, pheromones are picked up by the vomeronasal organ (VNO), located above the roof of the mouth. The VNO will receive a pheromone molecule and then send a corresponding signal to the brain. But for humans, VNO, while still present, isn’t actually functional

Diagram of the location of the vomeronasal organ (VNO) in humans

Location of the vomeronasal organ in humans (Image source: Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body)

Of course, a non-functioning VNO doesn’t mean that we don’t use pheromones at all—there has been a decent amount of exploration towards finding pheromones that humans might use to attract each other, although scientists have not been able to isolate any specific molecule(s) so far. Particular focus has been dedicated to the hormones found in our sweat. For example, a 2013 study found that androstadienone, a chemical in male sweat, was linked to better mood and focus in women. This, in turn, can be linked to women’s sexual response. As for pheromones secreted by women, estratetraenol is a strong contender, as it was found to have an effect on men’s cognition in a sexual and romantic sense. 

2D structures of estratetraenol and androstadienone

Estratetraenol and androstadienone are two proposed human pheromones. (Image Source: Royal Society of Chemistry)

Furthermore, reaction to these two suggested pheromones are also influenced by factors such as sexual orientation. For example, it has been observed that homosexual men react similarly to heterosexual women in response to androstadienone, while homosexual women partly reacted to estratetraenol, similar to heterosexual men.

Overall, the hormones androstadienone and estratetraenol do indeed affect human behaviour in a way that is linked to attraction. But whether these hormones definitely cause or mediate attraction needs to be further examined.

Sniffing out the immune system

Interestingly enough, another theory states that your immune system—of all things—might be involved in determining whether we find someone’s smell attractive. In your DNA, there exists a certain group of genes called the major histocompatibility complex (MHC). These genes code for proteins that help your immune system identify intruders (bacteria, viruses, and other nasties). 

However, not only are there many different types of intruders that can infect humans (219 types of viruses alone!), some strains are also able to mutate very quickly due to their short reproduction time. Ultimately, this means that new types of intruders are constantly evolving all the time, while our immune systems are also forced to adapt in tandem with them, which is known as an “evolutionary arms race”.

The Science of Smells

All sorts of nasties are looking for ways past our immune system and into the body.

An aspect of making sure our immune system can keep up with all those pesky pathogens is ensuring that the MHC proteins can correctly detect a wide range of intruders. One method is reproduction: if you have taken biology (or just read about it) before, you may recall that half of your genes are inherited from each parent. Technically, humans carry two copies of the human genome in their cells, for which the genes are identical. However, these genes can have alternate forms known as alleles, which is what makes us so diverse—it causes variation in hair or eye colour, or more importantly, in this case, variation in our MHC genes. Therefore, we can also inherit different types of alleles in our MHC genes. 

This means that if one parent had a gene that made their MHC complex better at detecting a certain virus compared to that of others, then the offspring would be able to inherit this ability from just one parent. This combination of different alleles could also create new variations that help the MHC complex detect new intruders, or detect them better! So to cover as many bases as possible, finding a partner with a very different MHC becomes more attractive, something which is supported by several studies on mice and humans. This forms the basis for attraction to partners with a different MHC. The method to find them, however, is to quite literally sniff them out, a phenomenon which is shared with a variety of animal species, including petrel chicks and stickleback fish.

The Science of Smells

A totally (un)realistic representation of what the immune system is doing (Image source: The New York Times)

And that was a ride! We’ve explored some science behind nice-smelling people, whether through the mysterious pheromone-signals or through practical immune system diversity. While science might not have all the answers yet, it can give you a bit of explanation— if you are someone who just happens to like how certain people smell, don’t think it’s uncommon, or strange or all in your head. Science has your back on this one!

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The Science of Smells
I'd make a chemistry joke, but all the good ones argon.