Sometimes we see the tears coming, and sometimes they catch us off guard; we find ourselves weeping without knowing why. It's a personal problem, but it's a scientific one, too: Why do people weep? What purpose does it serve? One expert attempts to answer these questions in a new article in the journal New Ideas in Psychology.

Article author Carlo V. Bellieni is a pediatrician and a bioethicist at Siena University Hospital in Italy. His previous studies have focused on children's emotional well-being and babies' crying and pain. For his latest paper, he examined data and observations on weeping from more than 70 studies and books from researchers stretching back all the way to Charles Darwin.

His conclusion? Weeping is "a complex phenomenon."

For starters, Bellieni writes, weeping is similar to crying, but it's not the same thing. Crying is typically a reaction to pain or anger. It's audible and physical, increasing heart rate, affecting breathing, and contorting the face and body. A crying person's voice changes, and their body makes more stress hormones like adrenaline. And while they don't shed tears, other animals cry, too.

Weeping, on the other hand, appears to be uniquely human. It's what happens when the cup of our emotions runneth over. We cry when we drop a cinderblock on our foot. We weep at funerals, and at weddings.

As Bellieni discovered, there are many theories on how we cry and weep, and where the tears come from. Some researchers have argued that we make tears to return ourselves to the soothing, fluid environment of the womb. Others theorize that our bodies start extruding tears (and snot) to keep our nose and throat from drying out as our breathing intensifies. Darwin's hypothesis was that the tears are a byproduct of scrunching up our faces, including the tear-production glands.

None of these theories seem especially plausible, Bellieni writes. So for now, the answer to the physical question is, "We don't really know."

The emotional and social sides of the weeping equation are slightly more straightforward.

Weeping is a form of releasing intense emotion and physical tension. When we weep, we tell our body that it's okay to relax. This helps us reset our system, so to speak, and move on.

And seeing someone weep makes us want to help them, Bellieni says. Weeping makes other people want to help us. Visible sorrow is an opportunity to strengthen social ties. And among social animals like us, strong bonds mean a better chance of survival.

It's wrong to think of weeping as wimpy or weak, Bellieni says. In fact, it's "a strong behavior with positive effects on health and social interaction."

"In the light of these data," he concludes, "weeping appears to be a primal and important human behavior that deserves more attention."