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

Blogging on Peer-Reviewed ResearchDo 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, K.A., Gottfried, J.A. (2007). Subliminal Smells Can Guide Social Preferences. Psychological Science, 18(12), 1044-1049. DOI: 10.1111/j.1467-9280.2007.02023.x

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