Evidence for mimicry of expressions has come from research by Olaf Dimberg, a psychologist in Sweden, who measured the tension levels in the facial muscles of volunteers while they were shown photographs of various facial expressions. When the volunteers saw an angry face, for instance, their facial muscles mimicked the anger, often to a degree that was invisible but was measurable by electronic devices.

Dr. Zajonc suggested that shared facial expressions bring on identical emotions because facial muscles play a role in regulating blood flow to the brain. ''You both smile because you feel good and feel good because you smile,'' Dr. Zajonc said. Role of the Carotid Artery

In an article published in 1985 in Science, Dr. Zajonc pointed out that the carotid artery that supplies blood to the brain forks at the neck, sending out a major branch to the face. As the facial muscles tense and relax, they tend to act as tourniquets on the veins draining blood from various parts of the brain, indirectly regulating blood flow and temperature in various brain areas. Dr. Zajonc's theory holds that this process helps to determine which chemicals are released in the brain.

In this view, which Dr. Zajonc conceded was speculative, a given facial expression - a smile, say - triggers the brain chemicals that evoke the associated feelings, such as happiness. Thus, when people mimic their spouses' facial expressions, they also evoke the same emotions, thus empathizing all the better.

''Facial mimicry allows a truer empathy because it triggers the same inner state,'' Dr. Zajonc said. ''Couples can understand each other much better when this happens.''

One sign that such empathy has been going on, in Dr. Zajonc's view, is that people's fixed facial features begin to resemble those of their spouses as a result of sharing the same expressions so often. In support, he points to the finding in his study that those couples who were found to resemble each other most greatly after 25 years were also those who reported the happiest marriages. Some Experts Are Skeptical

Other experts agree that shared emotions could gradually sculpture the faces of a couple to become more similar, but they do not agree with Dr. Zajonc's explanation.