Smells we can't detect affect judgments we make about people

[Originally posted in December, 2007]

ResearchBlogging.orgDo smells have an impact on how we judge people? Certainly if someone smells bad, we may have a negative impression of the person. But what if the smell is so subtle we don't consciously notice it? Research results have been mixed, with some studies actually reporting that we like people more when in the presence of undetectable amounts of bad-smelling stuff. How could that be?

A team led by Wen Li believes that the judges might have actually been able to detect the odor, and then accounted for it in their response -- giving a face the benefit of the doubt when there's a hint of bad odor.

But odor detection is a tricky thing. Sometimes you're not sure if your milk or wine has gone bad, even after giving it a good whiff. The researchers felt that controlling the odors for a study would be the key to getting good results.

They first determined the odor detection threshold for each of 39 student volunteers. This was done by having each person sniff bottles containing progressively stronger solutions of three different compounds: Citral ("lemon"), anisole ("ethereal"), and valeric acid ("sweat"). The threshold was determined by when they could detect the odor. Then, for the actual experiment, bottles that were about 100 times more dilute were used.

The students took a good sniff of one of the four samples (plain air was used as a control), then pressed a button indicating whether they believed an odor was present. At this point, a picture of a face with a neutral expression was displayed on a computer screen. Viewers rated the face for likeability on a scale ranging from -10 to +10.

Despite the highly diluted solutions (with as little as 7 parts per trillion of an odorant), about half of students were able to detect the presence of an odor at levels statistically higher than chance. Still, only four students could pick the odorless bottle, and none could identify the other odors. But the students who could detect when an odor was present were clearly detecting something. Was there a difference in how they rated the faces? Here are the results:


When the volunteers didn't detect the odor at all, they rated faces as significantly more likeable when they smelled the pleasant lemon scent, compared to when they sniffed the unpleasant sweat odor (the difference between lemon and neutral and control scents was not significant). By contrast, the students who were able to detect odors showed no significant difference in likeability ratings, no matter what type of odor they smelled.

So when odors are truly subliminal -- when we can't consciously detect them at all -- they do affect our ratings of others. The authors argue that when we are conscious of odors, we attempt to account for them in our value judgments. In this case, viewers recognized that the odor was unrelated to the face they were rating, and could successfully account for that fact. When they weren't conscious of the odor, then processing probably occurred at a different level.

Li W, Moallem I, Paller KA, & Gottfried JA (2007). Subliminal smells can guide social preferences. Psychological science : a journal of the American Psychological Society / APS, 18 (12), 1044-9 PMID: 18031410

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It makes sense that when you are aware of something that might cause you to be biased, you will attempt to compensate - or even possibly overcompensate - for that bias. Interesting that the likability rating for the unpleasant detected odor seems to be slightly higher than the likability rating for air, which seems to be a sign of overcompensation.

Could this be extrapolated to social situations, where, for example, people attempt to overcompensate for what they believe to be personal biases (I'm thinking of behaviours that are designated as "polically correct")?

Re methodology? Were the volunteers asked which odours they found pleasant, or was it assumed that they preferred the lemon, didnt like the sweat or didn't care about the anisole? Different people have different tastes.

Dave - if we can't detect it, then it can't possibly have an impact. Shouldn't your headline read: Subliminal odors affect judgments we make about people?!

Sweat is more complicated than being just unpleasant. Something that smells like it, the valeric acid, could indicate the presence of another person and facilitate a bonding attitude -- explaining the likeability ratings when detected. It would be interesting to see this done with a foul smelling odor like rotten eggs.

I was thinking that the smell of sweat can be a sexual attractant for some people. Additionally, I associate lemon with things like cleansers and furniture polish - things that are cold, antiseptic and unfriendly.

My girlfriend is a graduate student in Wen Li's current lab, so after some consultation with her, I can actually speak to some of this research.

@Sam: In high concentrations, valeric acid smells less like sweat and more like an incredibly disgusting mixture of cheese and feet. When my girlfriend let me have a whiff of it in the lab, it didn't really facilitate any associations for me other than "Oh God, that's disgusting!" In this study, the concentrations of valeric acid were a bit lower than Li has used in her later studies, so the odor mixture may not have been as starkly negative. I don't think Li has published any social judgment work with these higher concentrations yet, though (although she may within the next year if all goes well!).

@Marcia: Denise Chen at Rice University has a line of research on the psycho-social factors involved in olfaction. One of her studies (Chen and Zhou, 2008) looks at how women encode and process male sexual sweat. I think that this line of research is still in its infancy, though.

A humorous story regarding perceptions.

Years ago my wife was working as an animal control officer and one day had a minor encounter with a skunk which was slightly perceptable. We stopped in a Home Depot and heard some comments from someone else in the aisle, ascribing the smell to the somewhat disheveled looking man about 20ft away rather than to the official uniform-clad woman standing next to them.

ahhh perceptions....

It's been long known that pheromones play a big role in mate selection. Wouldn't those be considered 'undetectable odors?'
From a Northwestern University study "Despite the fact that subjects could not consciously perceive these smells, the subliminal pleasant odor made faces appear more likeable, while the subliminal unpleasant odor made faces appear less likable. These findings strongly suggest that the brain utilizes odor inputs to guide social behavior even in the absence of conscious odor awareness."