A baby who sees his father burst into tears suddenly starts crying himself, his sad little face the very picture of misery.
Is this empathy? Or is it, as psychologist Andrew Meltzoff thinks, something less exalted, such as emotional "contagion"?
A slightly more evolved toddler watches her mother wince and yell "ouch!" after hitting herself with a hammer. The child picks up a teddy bear to give it to her mother. Now, that has to be empathy, right? The child not only knew what her mother was feeling, she had an appropriately compassionate response.
But, then, what about chimps? When two chimps fight, the loser is often consoled by a third chimp. But do chimps, or toddlers, really understand what they're doing? If so, are they acting from altruism, or a more selfish desire to calm their fellow creature so they feel less distressed themselves?
Empathy is what we all claim we want more of - from spouses, bosses, friends and harried doctors. But what is it, exactly? Does it truly aid healing to be understood? Do empathizers run the risk of burning out if they care too much?
Empathy is nothing less than "the unseen glue that holds civilization together," says Meltzoff, co-director of the Center for Mind, Brain and Learning in Seattle,
From an evolutionary point of view, we're probably hard-wired for empathy, which confers "selective advantage," allowing the young - and the species - to survive, says neurobiologist Steven Hyman, provost at Harvard University.
Empathy is important from a mental health point of view, too. Sociopaths are dangerous in part because their lack of empathy allows them to commit atrocious acts without remorse.
But empathy can get tricky.
For Robert W. Levenson, director of University of California, Berkeley's Institute of Personality and Social Research, empathy comes in three forms. "Cognitive empathy [is] knowing what someone is feeling," he says.
"Emotional empathy," he adds, is what most of us mean by the term, "feeling what someone is feeling." And compassionate empathy is doing something about it - offering a teddy bear or a kiss.
The capacity for empathy probably grows, at least in part, out of a baby's inborn ability to mimic facial expressions. Through this, the infant constructs a world view that says, in essence, that other people are "like me," Meltzoff says.
At UCLA's Neuropsychiatric Institute, Dr. Marco Iacoboni used a technique called functional MRI to obtain brain images of people observing and mimicking the facial expressions of others showing fear, surprise, disgust, anger, sadness and happiness.
He found that a specific set of circuits - including a tiny, island-shaped structure called the insula - "lights up" whether people are merely observing an emotion or trying to imitate it. It suggests that "the way you understand the feelings of others is through your own body."
The trick is to do this without drowning in the other person's feelings. A good therapist, says Dr. Paul McHugh, a professor of psychiatry at Johns Hopkins Medical Institutions, can experience a patient's feelings as if they were his own, but without accepting the patient's unrealistic assumptions.
For the rest of us, there are potential pitfalls. In some people, experiencing another person's pain can lead to "empathic over-arousal," says psychology professor Nancy Eisenberg of Arizona State University in Tempe.
Empathy can even be too much for some people seemingly in need of it, she says: "Empathy could lead someone to feel they are being pitied, especially if that person has a strong need to be autonomous."
But most of the time, we all want more empathy, not less.
Judy Foreman is lecturer on medicine at Harvard Medical School. Her column appears every other week. Past columns are available on www.myhealthsense.com.