When gratitude comes from the heart, it not only keeps social relationships intact but can physically heal bodies, according to a new book that shines a spotlight on the emotion.

Edited by Robert Emmons of the University of California, Davis, and Michael McCullough of the University of Miami, "The Psychology of Gratitude" (Oxford University Press) looks at the emotion from perspectives ranging from the early Greek philosophers and religious scripture to modern primatologists, psychologists, biologists, philosophers and neuroscientists.

"Gratitude is a basic universal emotion, but it doesn't fit the typical model that emotions like anger, jealousy or fear do," says Emmons, a psychologist who has been studying the effects of gratitude on people's happiness and life balance for the past six years.

Unlike those other emotions, which are matched with universal facial expressions and corresponding physiological patterns, such as increased heart rates, gratitude can't be recognized immediately by other people.

However, although it's not visible, biologists can measure psychophysiological connections to the emotion. For instance, cultivating positive emotions such as appreciation and gratitude can improve the heart's rhythmic functioning, say contributors Rollin McCraty of the HeartMath Research Center and Doc Childre of Quantum Intec Inc.

A well-functioning heart, in turn, sends signals to the cognitive and emotional centers in the brain. McCraty and Childre say by practicing positive emotions like gratitude, people can heal physically.

Gratitude is also a very complex, if subtle, emotion that requires human beings to figure out when people are being genuine, say anthropologists Kristin E. Bonnie and Frans de Waal of Emory University, pointing to the necessary evolution of human social relations so that gratitude functions.

Bonnie and de Waal say gratitude requires a mutual give-and-take and the ability to be altruistic in this reciprocal effort. It also "demands advanced cognitive skills for recognizing partners, detecting cheaters and mentally keeping score," they write.

Although humans may be the only creatures with the full-blown cycle of indebtedness and gratitude, the two conclude that some primitive forms of the emotion may be found in animals.

Described as a sense of wonder, thankfulness and appreciation for life, the nature of gratitude has been debated for more than two millennia. Aristotle, for example, believed that the emotion was a weakness and incompatible with magnanimity because it is demeaning to be indebted to others.

It also is the basis of the Judeo-Christian moral teachings, point out book contributors Dan McAdams of Northwestern University and Jack Bauer of Northern Arizona University. "If the source texts for Christianity and Judaism are to be believed, ingratitude is what got us into trouble in the first place," they say, pointing to seminal stories such as Adam neglecting to thank God for the gift of Eve.

Philosopher Robert C. Roberts of Baylor University contrasts the positive virtues of gratitude with three significant emotional sources of unhappiness and dysfunction -- resentment, regret and envy.

"Grateful people tend to be satisfied with what they have and so are less susceptible to such emotions as disappointment, regret and frustration," he says, adding that these happier people are more likely to maintain good friendships and civility.

"The justice of gratitude can be plausibly argued to be metaphysical -- a kind of attunement to one's basic human nature and the nature of the universe -- because we are in fact dependent recipients of good things both from our fellow human beings and God," Roberts concludes.

The study of positive emotions such as gratitude is a fairly new field in psychology, started in the late 1990s to look at human strengths and virtues. One of the field's founders, Martin Seligman of the University of Pennsylvania, says, "The science of gratitude, led by Emmons and McCullough, has become a central focus of positive psychology over the last five years."

Emmons' next project is to research how gratitude is developed within children.

"What is exciting about gratitude is that it contributes not only to individuals but collectively to society. It's a civic virtue," Emmons says.