Unhappy couples become "physiologically linked" when discussing a problem in their marriage. Their hearts beat at the same time, they move simultaneously and even sweat together.

The smiles and frowns of infants as young as 10 weeks -- typically attributed to gas pains -- are actually the expression of their inner feelings.

Some people -- including, apparently, President Reagan -- can use their emotions to elicit positive physiological responses in others. Their smiles causes others -- even detractors -- to relax.

Those were among the dozens of study findings reported last week at the inaugural meeting of the International Society for Research on Emotions. More than 100 researchers from China, Europe and the United States gathered at Harvard University to discuss topics such as:

* How many emotions are there?

* What is the difference between a mood and an emotion?

* When do infants develop emotions?

* Are animals able to experience the same emotions as humans?

* What effect do emotions have in promoting and preventing illness?

"Emotion research is one of the most rapidly growing areas in behavioral sciences," says psychologist Paul Ekman of the University of California at San Francisco, who likens the research boom to the explosion in physics' studies half a century ago. "People are mobilized, the work is beginning and the biggest change is that we now have the tools for measuring emotion.

"What we know is that emotions are terribly important in people's lives. From how we raise our children to how to make more successful marriages to how we deal with the problems of violence, drug abuse and entertainment, there is hardly an area of life of any import in which emotions don't play an important part. We're not just talking about mental illness and physical disease, but what television program you're looking at to why they are hijacking airplanes." "Emotions have a lot to do with survival," said neuroscientist Candace Pert in a keynote address on the biochemistry of emotion. "Emotions get an animal to operate in an organized way."

Amid the constant barrage of challenges from the environment, said Pert, who is chief of the brain biochemistry division at the National Institute of Mental Health (NIMH), emotions "allow us to perceive what's important."

Pert's work is unraveling what researchers call the "substrate of emotion" -- a kind of biochemical foundation of feelings. Key to this new picture has been the discovery within the last five years of a group of chemicals called neuropeptides, which help shape sight, sound, smell, taste and touch -- the five senses that form the basis for emotional experiences.

Included among the neuropeptides is a group of chemicals known as opiates which are similar in structure to the drugs opium, heroin and morphine. These naturally occuring opiates help suppress pain. They're involved in eating, appetite control and probably produce the so-called runner's high.

Among the recent and diverse research findings of this emerging field are:

* A way to predict happiness in marriage. Unhappily married couples become "physiologically linked" when they're discussing a problem in their marriage, report Indiana University psychologist Robert Levenson and psychologist John Gottman of the University of Illinois.

"Their hearts beat at the same time," says Levenson. "They sweat at the same time, they move at the same time. In a way, they physiologically become what they are emotionally."

The researchers performed two separate studies -- one of 30 couples, the other of 80 couples -- sponsored by NIMH. They found that this physiological synchrony -- plus the level of emotions a couple experienced while trying to work out a problem -- were important predictors of marital happiness or dissatisfaction three years later.

Couples in the study completed extensive questionnaires about their marital happiness, then later went to a laboratory for physiological tests. For the eight hours prior to testing, all couples agreed to have no contact. In the laboratory, they were hooked to equipment that measured five physical responses including pulse, sweating and muscle movement. During a five-minute baseline measurement, husband and wife were in the same room, but were still not allowed to speak. Then for 15 minutes, the couple discussed daily activities while television cameras recorded the encounter.

Later, the couple chose a minor problem to discuss before the cameras, again for 15 minutes.

The more aroused couples became during their encounters, "the more likely their marital satisfaction was likely to decline over the next three years," Levenson says. When the couples were looked at again three years later, this measure proved to be correct 60 percent of the time in predicting which couples would be more dissatisfied, the researchers report.

One footnote, Levenson says, is how emotions can often be reexperienced at the same intensity by all married couples. More than 90 percent of participants showed the identical changes in heart rate, sweating and other measures when they reviewed the tapes a few days later without their spouses present.

* Growing evidence that infants experience and perceive a far wider range of emotions at a much earlier age than ever before realized. Ten-week-old infants are able to tell the differences in their mothers' expressions of sadness, anger and happiness, indicate studies by developmental psychologist Jeannette Haviland of Rutgers University. Moreover, infants express their own feelings at this early age by making their own facial expressions, not as a result of having gas or merely mimicking an adult's expression.

Ten-month-old infants show distinctive changes in brain wave activity with different facial expressions, reveal studies by University of Maryland psychologist Nathan Fox. In two studies sponsored by the National Science Foundation, Fox and psychologist Richard Davidson from the State University of New York at Purchase measured brain wave changes in infants by taking electroencephalogram (EEG) measurements. Their results suggest that "emotional expression is occurring in the frontal region of the brain."

Babies who cried when their mothers left the room showed an increase in frontal brain activity, while those who didn't showed a decrease in frontal brain activity. Other studies of infant smiles showed that brain EEG patterns looked one way when infants smiled at their mothers and another when they smiled at strangers.

These results suggest, Fox says, that "by 10 months of age, infants are already regulating their social communication."

* Evidence that President Reagan's political appeal may be related to his ability to generate good physiological responses. An individual's "prior attitude doesn't influence physiological responses to Reagan," report Dartmouth psychologists Robert Kleck and John Lanzetta.

"When he smiles, people can't help but respond," Lanzetta says. Reagan evokes positive physiological responses, Lanzetta believes, because -- unlike many people -- "his voice, facial expressions and gestures" are all consistent with whatever emotion he is conveying.

The result is that when Reagan smiles, "he evokes relaxation, pleasure and nice feelings," Lanzetta says, and "a decrease in skin conductance" -- a measure of emotional state -- in others. Reagan's smiles also are associated with a decrease in the autonomic nervous system -- part of the nervous system that controls stress.

People experience these physical effects from Reagan, Lanzetta says, regardless of their "prior attitude toward Reagan." But if they happen to be Reagan supporters, they feel these expressions "even stronger."