A Tear

The secret messages of smells.

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The latest episode of Glee brought viewers yet another tearfest: Quinn quivering tearfully when boyfriend Sam breaks up with her; Sue blinking away tears while singing “This Little Light of Mine” to hospitalized children; Rachel tearing up after a compliment from ex-boyfriend Finn; and a Justin Bieber serenade driving some glee club members into an emotional, teary frenzy.

   

It was enough to make a person wonder: Do tears serve a purpose beyond the obvious expression of emotion?

A study published recently in Science takes us a little ways toward an answer. Investigators found that the smell of tears triggered neither sadness nor empathy in men, but it did make them feel less attracted to women. Men exposed to the scent of fresh emotional tears experienced decreased testosterone levels and less activity in brain areas involved in sexual arousal.

The research was the first demonstration that the smell of tears can affect behavior, but it’s not the first time that research has found subtle messages transmitted through odors. We now know that babies can distinguish their mother’s scent — including the smell of their breast milk — from that of others. Scents emitted by breastfeeding women and their infants can induce other women to want to reproduce. Body odor can reveal when one person is physically attracted to another. We may even choose a mate whose body odor indicates a favorable genetic makeup. And science has proven that we really can detect the scent of fear in human sweat.

But while milk and sweat have a noticeable scent, tears, by all accounts, do not. Their power, according to the investigators from the Weizmann Institute, must come from odorless chemical signals that pass through the nose, are read by the brain, and are translated, unconsciously, into human behavior. Regular salt water does not have the same effect: Study participants who were exposed to the control “scent” of saline did not experience the same diminishment in arousal as the men exposed to real tears.

The real tears employed in the study were fresh, emotional tears, taken from female study participants who cried during viewings of sad movies. Emotional tears are not the same ones that result from cutting an onion, nor are they the ones that keep eyes lubricated so they don’t feel like sandpaper with every blink. No, they’re the kind of tears that spring, unwittingly, from your eyes when you hear a sad song, miss your mom, get criticized by your boss, or fight with your lover.

They’re the embarrassing type, usually fended off with apologies, and scientists have long puzzled over their evolutionary purpose while believing them to be uniquely human. Now, equipped with the new knowledge that they could transmit a chemosignal, it may be time to reevaluate whether they are uniquely human after all. Rodent tears are known to contain such chemical signals, for instance, said senior investigator Noam Sobel.

This question is just one of many that the research raises. Sobel and colleagues also do not know which chemical signal is contained in these tears, whether different kinds of emotional situations send different tear-encoded signals, or whether women’s tears are different from men’s tears or children’s tears.

Nonetheless, the study highlights that chemical signaling affects our behavior in ways that go completely unnoticed. And that “completely unnoticed” aspect begs the question: What other yet unrecognized chemosignals influence our behavior, while we are unaware? Smell, perhaps the least appreciated of all the senses, increasingly appears to play a pivotal role in how we live our lives. We can only hope that, with further research, we will better understand its mysterious, compelling ways. • 17 February 2011

REFERENCES: Human tears contain a chemosignal. Gelstein S, Yeshurun Y, Rozenkrantz L, et al. Science. 2011;331:226-30. Mother-infant skin-to-skin contact after delivery results in early recognition of own mother’s milk odour. Mizuno K, Mizuno N, Shinohara T, Noda M. Acta Paediatr. 2004;93:1640-5. Social chemosignals from breastfeeding women increase sexual motivation. Spencer NA, McClintock MK, Sellergren SA, et al. Horm Behav. 2004;46:362-70. Encoding human sexual chemosensory cues in the orbitofrontal and fusiform cortices. Zhou W, Chen D. J Neurosci. 2008 Dec 31;28(53):14416-21. Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity? Wedekind C, Füri S. Proc Biol Sci. 1997 Oct 22;264(1387):1471-9. Human olfactory communication of emotion. Chen D, Haviland-Jones J. Percept Mot Skills. 2000;91:771-81.

Jennifer Fisher Wilson is the science reporter for Annals of Internal Medicine. Her stories are available at www.annals.org.

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