We can alter the shapes of our bodies, slow the signs of aging and learn to control our heart rates. Yet we're often powerless when it comes to crying.
Some new research efforts are helping to piece together the biological and cultural forces behind crying, showing that there are different types of tears as well as differences in the way men and women cry.
Women are biologically wired to shed tears more than men. Under a microscope, cells of female tear glands look different than men's. Also, the male tear duct is larger than the female's, so if a man and a woman both tear up, the woman's tears will spill onto her cheeks quicker. "For men and their ducts, it'd be like having a big fat pipe to drain in a rainstorm," says Louann Brizendine, a neuropsychiatrist at the University of California, San Francisco.
Social conditioning comes into play in restraining the impulse to cry, Brizendine says. When we experience physical pain or emotional sadness or frustration, the brain's amygdala, which is part of the limbic system or "emotional brain," fires up signals. If the stimulus is great enough, the energy can travel from the emotional area into the frontal motor strip. That's when breathing can devolve into sobbing.
Boys often come up with mechanisms to calm themselves before they cross the precipice from tearing up to weeping.
"Boys are taught over and over again not to cry: to scrunch their faces, to think about the Gettysburg address, to distract themselves," says Dr. Brizendine, the author of the best-selling book, "The Female Brain."
Research indicates that testosterone helps raise the threshold between emotional stimulus and the shedding of tears. "It helps put the brakes on," she says.
To teach acting students to cry at the New York Film Academy, Glenn Kalison reminds them to consider that since they were babies, they have been building barriers to prevent themselves from crying. The trick is to imagine a character's pain and sadness, and then to connect with the barriers that character would have built, he explains. The realistic way to portray crying isn't to let tears flow, but to show the struggle not to cry.
"It's the attempt at suppressing the crying that is the familiar sensation," says Mr. Kalison, chairman of the academy's acting for film department. "Only actors want to cry."
Studying tears and the process of crying is complex. There are two types of tears. Irritant tears help wash eyes of dust, dirt and impurities. Emotional tears are created and released in response to emotional stimulus and physical pain. All tears contain proteins, salt and hormones, among other substances, but emotional tears have higher levels of protein, says William H. Frey II, a neuroscientist and biochemist at Regions Hospital in St. Paul, Minn., who conducted research into the composition of tears.
One hormone in tears is prolactin, a lactation catalyst. Just as it helps to produce milk, prolactin also aids in tear production. By the time women reach 18, they have 50 percent to 60 percent higher levels of prolactin in their bloodstream than men do.
"We believe this is one of the reasons that women cry more easily," Frey says.
Much remains unknown. Human beings are the only species that cries emotional tears, making it difficult to study the internal mechanics of tear glands. It is also not that easy to stimulate crying in a controlled research environment.
Ad Vingerhoets, a professor of clinical psychology who focuses on stress and emotion at Tilburg University in the Netherlands, says that to elicit tears, he shows research participants photographs of people in various states of crisis to try to elicit tears of empathy. He also shows tear jerkers, like the 1979 movie "The Champ."
"We do not poke people in the eye," Dr. Vingerhoets says. "We use onions." Test tubes and vials are held to people's cheeks to collect the tears.