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Key points
- Tears emit a social chemical signal that is picked up by the olfactory system and lowers aggression in males.
- Chemical signals in tears strengthen the connection between the aggression and olfactory parts of the brain.
- This study suggests that tears are a chemical means of protecting against aggression among mammals.
Humans have a far smarter sense of smell than is commonly understood. The human olfactory system picks up chemical signals, or chemosignals, emitted by other mammals that don’t have a perceptible scent, including tears. A recent study has found that emotional tears emit a social chemosignal that lowers male aggression and strengthens the connection between the brain regions responsible for olfaction and aggression. These findings contribute new understandings of the complex interplay between chemical signals, neural responses, and social behavior in humans with the potential to influence emotional and social interactions.
Previous studies have established the visual impact of tears as clear signals of distress in adults, eliciting sympathy and diminishing irritation among observers. What’s less widely understood is the imperceptible impact of chemosignals in emotional tears that are picked up by the olfactory system. The study, published last month by Drs. Shani Agron and Claire A. de March in PLOS Biology, adds to this body of research, further elucidating the social and behavioral impact of chemosignals in tears and suggesting that tears protect against aggression.
What Are Social Chemosignals?
Social chemosignals are the chemical signals transferred by different species and identified through the senses, including the sense of smell or olfaction. Those signals have a strong yet undetectable impact on human emotions and social behavior. For example, one study found that humans exchange chemosignals during handshakes and will often overtly sniff their hand following a handshake, subliminally searching for chemosignals that will provide information on the person they interact with.
Another study found that the chemosignals emitted from fear sweat improved cognitive performance, suggesting that humans act with greater caution when exposed to fear chemosignals. Social chemosignals serve an important role in mediating and influencing behavior, transcending cultural and behavioral norms or expectations.
New Research Discovers Chemosignals in Human Tears
Previous research identified chemosignals in rodent tears that decrease aggression in male rodents. Researchers at the Weizmann Institute of Science in Israel asked whether human tears contain the same chemosignals. For their recent study, Drs. Agron and de March measured the aggression of male participants playing an online game while sniffing either tears or saline, which was used as a control substance. Following exposure to tears, there was a 43.7% decrease in aggression, suggesting that there is a “stop aggression” signal in human tears, as has been previously found in rodents.
In their second experiment, the researchers asked whether the human olfactory system can process chemosignals in tears. They exposed 62 human olfactory receptors (ORs) in cells to either tears or saline. They found that four of the 62 ORs were activated by tears and not the trickled saline, demonstrating that despite being perceptually odorless, human emotional tears send a signal that generates a brain response through the main human olfactory system.
The final experiment aimed to understand the brain response to sniffing tears in the context of aggression. Using fMRI on participants while they were exposed to tears or saline while playing the aggression game, the researchers found that two regions previously implicated in aggression saw significant decreases in activity while sniffing tears. The researchers then observed strong connections between the brain networks associated with aggression and olfaction, concluding that tears decrease activity in the brain regions responsible for aggression and increase the connections between the brain regions responsible for aggression and olfaction.
The current research has focused on the chemosignals found in female tears; the specific chemical composition of male tears have not been studied. Further research needs to be conducted to determine whether male emotional tears have the same effects as female tears. Nonemotional tears, like those caused by onions or tear gas, do not contain the same chemical composition as emotional tears and are not associated with the same emotional signaling. Nonemotional tears are a result of foreign particles or substances irritating the eye and are not a form of emotional communication.
Relevance and Future Impact of Chemosignals in Tears
While this study merely looked to determine the presence of chemosignals in human tears rather than explore why humans may have them, the authors speculate that as crying often happens in proximate interactions, chemosensing in tears may serve as a subliminal protection message. They also note the relevance of chemosignals in infant tears as they may serve as a defensive mechanism for infants who don’t have other means of protecting themselves.
The identification of the chemosignals in tears could have various future applications. Synthesized versions of the identified chemical signal could reduce aggression in conflict-resolution settings and promote calmer discourse. It could also be included in various therapeutic interventions related to anger management or relationship counseling. There’s also the potential for knowledge of chemosignals to be manipulated for marketing and product development, perhaps creating products and services to reduce aggression.
This study adds exciting new findings to the growing body of research exploring chemosignaling in humans, providing interesting insights into how human behavior can be influenced by subconscious chemical messaging.
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