Animals aren't the only one who use their sense of smell to sniff out people they like, for humans also do so.
The finding is based on a study led by Wen Li, a post-doctoral fellow in the Cognitive Neurology and Alzheimer's Disease Centre at Northwestern's Feinberg School of Medicine, which stated that minute amounts of odours elicited salient psychological and physiological changes, which suggests that humans get more information from barely perceptible scents.
"We evaluate people every day and make judgments about who we like or don't like," said Li.
"We may think our judgments are based only on various conscious bits of information, but our senses also may provide subliminal perceptual information that affects our behaviour," Li added.
In the study, participants were asked to sniff bottles with three different scents: lemon (good), sweat (bad) and ethereal (neutral).
The scents ranged from levels that could be consciously smelled to those that were barely perceptible.
Study participants were informed that an odour would be present in 75 percent of the trials.
Most participants were not aware of the barely perceptible odours. After sniffing from each of the bottles, they were shown a face with a neutral expression and asked to evaluate it using one of six different rankings, ranging from extremely likeable to extremely unlikeable.
People who were slightly better than average at figuring out whether the minimal smell was present didn't seem to be biased by the subliminal scents.
"The study suggests that people conscious of the barely noticeable scents were able to discount that sensory information and just evaluate the faces. It only was when smell sneaked in without being noticed that judgments about likeability were biased," Li added.
The study's co-author Ken Paller, professor of psychology in the Weinberg College of Arts and Sciences at Northwestern said: "When sensory input is insufficient to provoke a conscious olfactory experience, subliminal processing prevails and biases perception. But as the awareness of a scent increases, greater executive control in the brain is engaged to counteract unconscious olfaction."
Jay Gottfried, assistant professor of neurology at Feinberg and senior author of the paper said that the acute sensitivity of human olfaction tends to be under-appreciated.
"In general, people tend to be dismissive of human olfaction and discount the role that smell plays in our everyday life," said Gottfried.
"Our study offers direct evidence that human social behaviour is under the influence of miniscule amounts of odour, at concentrations too low to be consciously perceived, indicating that the human sense of smell is much keener than commonly thought," Gottfried added.
The study adds to a growing body of research suggesting that subliminal sensory information -- whether from scents, vision or hearing -- affects perception.
"We are beginning to understand more about how perception and memory function by taking into account various types of influences that operate without our explicit knowledge," Paller said.
The study is published in Psychological Science.