Monday, January 31, 2011
I'm a little behind on this story. Not reading it but posting it. This one is all about tears and their effect on people. When you are near someone who is crying how does it make you feel? We know that seeing tears has an effect on people but is there something more to it than that? Is there a chemical component as well? After all, tears are glandular secretions.
Emotional tearing is a distinctly human behavior. Evolutionarily speaking, tears started out as an emotion-relevant function before turning into an emotion-signal alone. Charles Darwin suggested that one. If you think about it, it is a bit of a paradox. Tears serve as an emotional signal but do not serve an emotionally relevant function. Now keep in mind that, for the purposes of this article, we are not talking about tears as an adaptation to protect the eye or a mechanism to expel toxic substances. The authors of this study are looking for the functional significance of emotional tears.
Tears are produced by the lacrimal, accessory lacrimal, and Meibomian glands. The cornea (the transparent front part of the eye) is covered by a pre-corneal tear film which is replenished during blinking. This film creates a smooth surface, keeps the eye moist, helps supply oxygen to the eye, contains enzymes that prevent bacterial infections, and generally flush out the eye. The film is composed of three layers: oil (lipid), water (lacrimal or aqueous), and mucous (mucoid or mucin). The Meibomian glands (which line the edge of the eyelids) produce the oil, the lacrimal gland (lies underneath the outer orbital rim bone, just below the eyebrow) produces the water, and goblet cells in the conjunctiva produce the mucous. The lacrimal gland is the major producer of tears when a person cries.
It is known that the chemical makeup of human emotional tears differs from that of reflexive eye protective tears. The authors of this study took a closer look at some of those chemical differences, but not so much the actual components but their effects on the emotions/reactions of other people.
They had two female volunteers watch sad films in isolation and collected their tears. Separatately, they collected saline solution running down the same womens' cheeks to control for any skin-bound odor sources. In a blind test they presented the tears to 24 men to test whether they could smell a difference between the fresh tears and the saline. Next, they asked whether sniffing such odorless tears influences perception in order to see if tears contain a chemosignal related to a sadness context or if tears signal information related to sociosexual behavior. So they asked the male volunteers to again sniff a jar containing a compound (the tears or the saline), rate the intensity, pleasantness, and familiarity of the compound, and then with a pad of the compound stuck under their nose to rate the sadness and sexual attractiveness of a series of female subjects shown in a collection of photographs and answer questions that assess empathy.
The researchers found that tears did not differ from saline in perceived intensity, pleasantness, or familiarity - basically, they were odorless. They did not find a significant result in the ratings of sadness with tears. They did find that the ratings of faces differed after sniffing the tears or saline. A shift in the sexual attractiveness attributed to faces was observed. The men rated faces as less sexually attractive after sniffing the tears versus sniffing the saline. This result prompted the researchers to do another study. Perhaps the tears failed to influence sadness/empathy because the experimental context was not sad, and this was their chance to look at several other factors that influence these reactions.
The same procedure was followed for tear gathering. They tested 50 male volunteers using a paradigm that generates negative emotions. They measured psychophysiological arousal (galvanic skin response, heart rate, respiration rate, and skin temperature), self-ratings of mood, salivary levels of testosterone (before, during, and after sniffing the tears), and even did some brain imaging. Again, they found the tears to be perceived as odorless. The analysis showed that the men who sniffed the tears had a reduced self-rating of sexual arousal after a sad film, but this was only a moderate effect. The psychophysiology and hormonal responses, however, were pronounced. Several of the above listed arousal responses increased while sniffing the tears only to decrease dramatically afterwards. The men were also found to have lower testosterone levels following tear exposure, a significant indicator of reductions in sexual arousal in men. The brain scans showed that, in areas associated with sexual arousal, brain activity was lower after sniffing tears than after watching a sad movie.
All of these results together suggest that there is a chemosignal in women's emotional tears that reduces sexual arousal in men. When you consider that exposure to tears is, in many cases, in close proximity, at least in Western culture, it is not so difficult to believe. Just look at the act of comforting a crying loved one. In a hug you place your nose near a teary cheek and so will inhale some of these chemosignals. It is not usual for mammals, including humans, to use these signals, this is just the first study to show it.
Here's the paper:
Gelstein, Shani, et al. Human Tears Contain a Chemosignal. Science: 331 (6014), 226-230. (DOI: 10.1126/science.1198331)
And here are a couple of story links (although, this was picked up by a lot of news outlets so all you have to do is Google it to find more):