Yet psychologists and primatologists have been arguing for years that compassion is an evolved instinct, rooted in the brain’s circuitry. In a new book, “The Better Angels of Our Nature,” the psychologist Steven Pinker calls empathy “the latest fashion in human nature.” Chimpanzees show evidence of compassion, as do some monkeys; even mice seem to feel the pain of close peers. But if current trends continue, rats might become a more appropriate subject of study.

Are people today — are societies — really becoming somehow more callous?

The answer is no, of course not — at least not in any fundamental sense. But compassion is a limited resource, a system rooted in cognitive networks that tire and need refueling. And it’s not always rational.

Those in the so-called helping professions know as much about the limits of empathy as they do about its merits. Studies of oncology nurses, trauma workers and even marriage counselors, among others, have documented a common “compassion fatigue” that seems directly related to the amount of emotion shared. “In particular, listening to people who are suffering and not being able to do enough for them puts a tremendous weight” on caregivers, said Dr. Charles Figley, a psychologist at Tulane University.

In just the past decade, he said, professional organizations have begun to give guidelines to offset fatigue, like urging counselors to take time off, seek support from colleagues, even engage in therapy themselves.

Therapists quickly learn to recognize the signs.

Fatigue often results “when you’re seeing the same problems repeatedly, when they’re chronic, and when the outcomes are not good,” said Bret A. Moore, a former Army psychologist and co-author of “Wheels Down: Adjusting to Life After Deployment.” “One sign that you’re there is that you start hoping your appointments cancel.”