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Lots of animals yelp or cry out when they're in pain. But as far as scientists can tell, humans seem to be the only species that sheds tears for emotional reasons. As part of our series on evolution, The Human Edge, NPR's Alison Aubrey explores whether the uniquely human act of weeping gives us some advantage.

ALLISON AUBREY: Actors have a few tricks to help them cry on cue. There's the fresh-chopped onion, which'll really get the tear ducts going, and there are glycerin drops, a soap opera technique for coaxing those big, beautiful tears that roll down the cheeks.

That's because it's really hard to cry, unless you're really feeling something.

Ms. JANE DALY (Actress, Acting Instructor): Crying has to come from emotion, right? That's where we all cry from anyway.

AUBREY: Jane Daly is an actress, as well as a long-time acting instructor. She says tears come not only from grief or pain, they flow from all sorts of other emotions.

Ms. DALY: Joy, the birth of a baby, frustration. Tears of frustration - they can be cathartic and releasing. It's what makes us completely human.

AUBREY: Tears are universal. And think about it for a minute, we need them to keep our eyes lubricated. But why on earth should streams of salty drops spew forth from our eyes, blurring our vision, making our eyes puffy, when we get emotional.

Scientists who study evolution say crying probably conferred some benefits, did something to advance our species, because it stayed with us. Looking back at our early human ancestors, psychologist Randy Cornelius of Vassar College says his hunch is that crying may have evolved as a kind of signal - a signal that was valuable because it could only be picked up by those closest to us who could actually see our tears.

Professor RANDY CORNELIUS (Psychology, Vassar College): You can imagine that there would be selection pressure to develop a signaling system that wouldn't let predators in on the fact that you're vulnerable, but that would let your intimates in, you know, somebody within a couple of feet of you who can see what's going on and so then try to help.

AUBREY: To this day, tears can play an important role in communication. And the extraordinary thing is that tears don't just telegraph our state of mind to others, they can also evoke strong emotions in the people who witness them.

We've all experienced, at some point, that we not only cry from our own pain, but we're moved to tears by other people's sadness.

Mr. DALY: I like to use the word empath. I think a lot of people know the word empathetic or empathy. There's sympathy and then there's empathy. Sympathy, you can feel sorry, you know, for somebody going through something terrible. Empathy, or an empathic or empathetic person - i.e. a lot of actors - actually feel the darn thing.

AUBREY: Take Tom Hanks in the movie "Saving Private Ryan." He plays a World War II Navy captain. In one scene...

(Soundbite of movie, "Saving Private Ryan")

Mr. TOM HANKS (Actor): (As John H. Miller) You're going to be all right. You're going to be all right, John.

AUBREY: The company medic dies after desperate attempts to save his life. Hanks is overwhelmed.

(Soundbite of movie, "Saving Private Ryan")

(Soundbite of Tom Hanks crying)

Ms. DALY: He just breaks down sitting on an embankment on the side of a bridge at the loss, at another loss, of a skilled, wonderful young man who barely had a chance to live.

(Soundbite of Tom Hanks crying)

Ms. DALY: To see a grown man cry, it brings you to your knees.

AUBREY: And in that moment, we the audience feel it, too. Witnessing these tears, we get choked up. And this is because as humans, one of our signature abilities is that we're able to put ourselves in someone else's shoes to feel what someone else is feeling, predict how they might react - something academics call having a theory of mind.

Mr. JESSE BERING (Director, Institute of Cognition and Culture at Belfast University): So, a theory of mind is something that even four-year-olds have. A theory of mind basically means that you have a theory about the unobservable psychological states that are governing or causing other people's behaviors.

AUBREY: Jesse Bering directs the Institute of Cognition and Culture at Belfast University. He says, instinctively, we pick up on gestures - facial expressions, body language - and that's just the beginning. He says this power of empathy is huge, and it's fundamental to pretty much everything we do, from forming close relationships to living in complex societies. Bering says those of our early ancestors who had the most empathy, probably thrived because it helped them build strong communities, which gave them protection and support.

Within these communities, Bering says, tears could be powerful tools. They did more than just signal vulnerability - they were perhaps a way of keeping social and reproductive bonds strong. Maybe good criers were survivors.

Mr. BERING: Crying seems to elicit compassion and guilt, and that itself may be an evolved mechanism to save relationships that are in distress.

AUBREY: Bering says take his own recent experience. Not long ago, he found out that his partner had cheated on him.

Mr. BERING: I mean, it was devastating, the fact that he cheated on me. And I was convinced that I was going to end the relationship right there.

AUBREY: But many apologies into a long conversation, his partner began to tear up.

Mr. BERING: It wasn't sobbing, it was not a hysterical cry. He was trying to choke back, he was trying to hide the fact that he was crying. And, you know, that was only through this discussion about our relationship and, you know, how much I invested in it and how much he meant to me and how much he had hurt me. When I saw him cry, I realized that I was basically getting through to him.

AUBREY: Bering says he realizes that the tears did the heavy lifting here. The same conversation minus the tears may not have had the same effect.

And Bering says this illustrates another aspect of crying - whether it's intentional or unintentional, crying is a powerful way to get what you need or want.

Think about how babies get your attention - they cry. And there's some evidence that natural selection favored infants whose cries were most alarming.

Mr. BERING: And as a consequence, these babies, who, of course were our ancestors, would have been less likely to find themselves left alone or with strangers for long periods of time, and therefore less likely to befall genuine harm.

AUBREY: And crying can protect us throughout our lives. Just think how you react when someone starts to cry.

Mr. BERING: I mean, just think about how difficult it is to punish somebody, or to argue with someone who is crying. I mean, it's like this sort of trigger that tells us to back off on that individual.

AUBREY: And tears don't often lie. Think about Tiger Woods and his public plea for sympathy after his extramarital affairs came to light.

(Soundbite of press conference)

Mr. TIGER WOODS (Professional Golfer): I know I have bitterly disappointed all of you.

AUBREY: When he choked back tears, Men's Health editor Peter Moore says we all know where those tears were coming from.

Mr. PETER MOORE (Editor, Men's Health): They're kind of shameful tears. They're I'm a dope, look at what an idiot I am, pity me, tears. And I think that, you know, spare me. I'd just as soon draw the curtain over that one.

AUBREY: And maybe that's another reason evolution has kept humans crying: Tears help reveal the truth. And that's because along with the tears, we've evolved a very sophisticated ability to interpret them.

Allison Aubrey, NPR News.

(Soundbite of music)

MONTAGNE: You can hear more about how empathy gave humans the edge this afternoon on ALL THINGS CONSIDERED. This time, through the eyes of somebody who knows that her brain works differently - a woman with autism.

(Soundbite of music)

MONTAGNE: This is NPR News.

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