Emotions -- from anger and misery to elation and laughter -- are just as contagious as the flu.
Even strangers are capable of infecting us, as when someone honks insistently on the highway and other drivers feel their tempers rise. Couples who are married or living together are particularly vulnerable to "catching" each other's moods.
"Everybody picks up on the energy and emotions of others and is affected by them -- even if the effect is less than conscious," says Eric Morris, an acting coach in Los Angeles, who has used "infectability" as a technique in training actors, including Jack Nicholson and Arnold Schwarzenegger.
"Laughter is one of the most infectious kinds of expressions. If you start one person in a room laughing -- even if it's a somber group -- and keep that person laughing long enough, everybody else will eventually join in."
That kind of infection is positive, as is empathy -- sensitivity to some one else's feelings. But when you cross from empathy to symbiosis (living together) there are problems.
"In such cases, the boundaries merge," says Ellen McGrath, executive director of the Psychology Center in Laguna Beach, Calif. "You become the other person. Her feelings become your feelings, and that's not healthy."
Studies indicate women are far more susceptible to symbiosis than men.
"To help with the survival of the species, one of the sexes needs to be exquisitely attuned to the feelings of those around them -- particularly infants and children," Ms. McGrath says. "Women are still conditioned to be the caretakers of everyone else right from the start."
For women, perhaps the most contagious of all emotions is depression. A recent study from the American Psychological Association found women are twice as likely to get depressed as men.
In the face of a friend's unhappiness, try to be objective. You can observe the feeling without drowning in it, which also best serves the person you are trying to help. Ask yourself, "Whose problem is this?" If it's not your problem, just listen.
Contagious states like depression often need more than an open ear. One effective antidote is taking action. Ms. McGrath advises that you "set a mental time limit on how long you'll talk over the problem."
Toward the end of that time, try to get the friend to stop dwelling on her woes by suggesting doing something -- go for a run, spend the next day gallery-hopping. Even rearranging furniture can alter perspective.
"Then the support isn't just emotional," Ms. McGrath says. "It's behavioral."
People who have the most difficulty separating their own feelings from those of others are like chameleons, changing colors depending on who's around them.
"If we're lucky, by the time we're little kids, we learn that we're separate," says psychologist Irma Rahtjen of Kansas City, Mo. "We discover we can live without Mommy for an hour or two because we know she'll come back. It's the kid who doesn't know if Mommy will ever come back who has trouble developing boundaries."
Counseling, such self-help groups as Emotions Anonymous, even acting classes, can provide a reassuring environment to help chameleons unfurl their own unique colors.