Shape of odour molecules determines smell

Our preference for smell feels personal. We have our favourite colognes, soaps, and food scents. But do we like certain smells because everyone around us likes them and so that’s the way we’ve grown up. The answer’s no! It’s apparently not a cultural thing; people the world over like - and loathe - the same sorts of smells, as Julia Ravey heard from Asifa Majid from the University of Oxford…

Asifa - Smell studies like most psychology studies, or medical studies more broadly, typically focus on Western participants, usually living in big cities, usually near universities, usually in the middle classes. When we're making theories about how things work, we hope to be making claims about the whole of the human population. So, you really want to be testing people from diverse backgrounds, especially if we want to try and untangle whether it's culture that, for example, causes our preferences for smells, or just individual preferences, or biology.

Julia - Which communities did you study?

Asifa - We tried to get people with very different lifestyles. So, we tested different hunter gatherer communities. And then we had some small scale farmers in Ecuador, for example, and then people that live in big cities in Mexico and Thailand. So, we're trying to take people with very different sorts of experiences.

Julia - When ranking these different smells, what came out on top and what was the least liked smell?

Asifa - In our study, people across the world all really liked the smell of vanilla and they least liked the smell of isovaleric acid, which smells a little bit like sweaty feet. It didn't matter what culture people were in or what kind of environment they lived in, everybody generally preferred vanilla and they all thought that the isovaleric, sweaty feet smell was really bad.

Julia - How much influence did culture actually have in these preferences?

Asifa - A really small amount. Only 6% of the data could be explained by culture. Most of it seemed to be shared, and then there were some individual differences too, but culture plays a very small role.

Julia - How much of the proportion of what people preferred in terms of smell came down to the molecular structure of the odour molecule?

Asifa - Anything that we smell is basically a molecule, and any molecule has thousands of different features. We can look at those features and see if we can predict how pleasant something is, and, when we do that, it turns out we can predict, just from the molecule itself, which ones people are going find nice and which ones they're going to find stinky.

Julia - If we're genetically built to prefer certain smells over others, why do you think that is?

Asifa- Some smells are probably telling us that something's dangerous, so our sense of smell is very sensitive to things that could be going wrong. We can smell, for example, a gas leak before we can see anything going wrong, we can smell fire from a distance, or the fact that the milk is going off. Our noses are very sensitive to signals of danger and it's likely that what we're finding is the same sensitivity to things that could be potentially toxic for us. I think COVID has taught us that the sense of smell is really important. One of the symptoms that we know that COVID has is a loss of the sense of smell, but luckily the sense of smell is really malleable and flexible, so people do have their sense of smell returning.

Julia - Yeah, I think we take our sense of smell for granted. When I had COVID and lost mine, it was so acute. You don't realise how much pleasure and joy smell brings into your life until it's taken away from you.

Asifa - Absolutely, yeah. So everybody should go out there and sniff something and enjoy it.