Crying Science: Why Do We Shed Tears When We're Sad? (VIDEO)

WATCH: Why Does Sadness Make Us Cry?

We're all too familiar with tears welling up in our eyes--brought on by stress, sadness, or even laughter. But why exactly does feeling things make liquid come out of our eyes? And why is that uniquely human? After all, we're the only species that sheds emotional tears.

To unravel the science behind sobbing, I spoke with expert Dr. William H. Frey, who also serves as director of the Alzheimer's Research Center at the HealthPartners Institute for Education and Research in St. Paul, Minn. He explained that not all tears are the same.

Check out the video above and/or click on the link below to learn more. And don't forget to leave your thoughts in the comments section at the bottom of the page. Come on, talk nerdy to me!

CARA SANTA MARIA: Hi everyone. Cara Santa Maria here. Crying is a pretty weird behavior, no? Smoky rooms and strong wind makes us teary-eyed. And chopping onions releases sulfur, which makes sulfuric acid when it hits our wet eyeballs. If you get poked in the eye or an eyelash is caught under your lid, ugh, that's the worst! Definitely gets me teared up! But why do strong emotions like sadness or frustration make water come out of our eyes?

WILLIAM FREY: This is the only physiological function that humans have that other animals don’t have. We have a lot of cognitive abilities and mental abilities that other animals don’t have but when it comes to actually a bodily secretion and excretion, this is the only thing we have, emotional tears, that other animals don’t have. And it’s there for a reason.

CSM: That's Dr. William H. Frey. He's the director of the Alzheimer's Research Center at the
HealthPartners Institute for Education and Research in St Paul, Minnesota. He wrote the book on tears, literally! It's called Crying: The Mystery of Tears. He says not all tears are the same--there are actually three different kinds. Basal tears are sort of always present. They keep our eyes from drying out. Reflex tears fill our eyes when we're in a smoky room, when we're chopping onions, or when an eyelash gets in there. And appropriately named emotional tears are shed when we' know...emotional--and about anything, really. Even physical pain.

WF: For a long time it was thought that all tears were basically the same and then we conducted some studies and we showed that emotional tears were different from those that you might produce in response to eye irritation by onions for example. And what we reported in the American Journal of Ophthalmology was that emotional tears have a higher content or concentration of protein.

CSM: Specifically, stress hormones like prolactin and adrenocorticotropic hormone. Some even hypothesize that crying is the body's way of shedding these hormones to literally reduce your levels of stress. But I'm not convinced. I haven't seen any good evidence to support this idea, and who's to say that leaky hormones isn't a by-product of the fact that circulating levels are high when we're emotional? Oh, and would you believe that scientists have actually studied how tears affect sexual arousal? Turns out that crying is not sexy. Men who smelled female emotional tears showed reduced levels of sexual arousal in multiple studies. Also, emotional tears aren't reflexive, like the other types. Our lacrimal glands have a neural connection to the limbic system of the brain, the structures that work together to process emotional information. So okay, this tells us how we cry, but why do we do it?

WF: We do know from our studies of many men and women, adult men and women, that people feel better after crying. Eighty-five percent of women and 73 percent of men report feeling better. People feel less angry, less sad, so crying does appear to alleviate stress.

CSM: Researchers also found that when subjects were shown photos of people crying alongside photographs where tears had been digitally removed, they (surprise, surprise) rated the tearful portraits as more sad. The researchers concluded that “emotional tears resolve ambiguity." This may seem obvious at first, but think about it. With tears, more sad. Without them, less sad. See, evolutionary psychologists think that tearfulness may have evolved as a strong emotional cue--a signal to others that you're upset, in pain, need help--especially before we developed language. Those who cried and felt empathy when others cried may have had an evolutionary advantage.

Perhaps criers bonded the group, helped build communities and social support, and eventually this trait found its way into all of us. And because tears aren't easy to see from far away, they may have evolved to tell your friends you're in need, but not your enemies. We wouldn't want them to know we're vulnerable, now would we? So, when's the last time you had a good cry? How did it make you feel? Let me know on Twitter, Facebook, or leave a comment on The Huffington Post. Come on, talk nerdy to me!

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