Children under the age of 3 can suffer from symptoms of depression, including disruptions in eating and sleep. In recent years, researchers have discovered the youngest humans can even suffer from post-traumatic stress disorder, once thought to be only an illness of adults.

"The picture has totally changed," says Alicia Lieberman, director of the Child Trauma Research Project at San Francisco General Hospital.

Although much of psychology is built upon the influences of childhood, psychologists haven't always paid much attention to the earliest years of a child's life. Only in the late 1960s and 1970s did researchers begin to understand the importance of the relationships between infants and those who take care of them, says Alice Sterling Honig, professor emerita of child development at Syracuse University.

Researchers watched how infants reacted when their parents went to the hospital and found signs of trouble. "First, the little babies would protest enormously and search around frantically," Honig says. "But after a while, they'd go into a despair and withdraw and look listless, with dull eyes, as if they gave up looking for their special person."

Parents who continually fail to create a bond of trust with their babies may doom them to lives of insecurity, Honig adds: "You're not going to have this feeling of trusting that someone is really for you. There's a lot of continuity from infancy all the way to people who [grow up] and ask: 'Do you love me?' 'How come you didn't call me yesterday?' And 'I saw you looking at that woman!'"

Psychologists, of course, can't ask infants how they feel. "We don't put babies on couches," Lieberman says. Instead, they rely on instinct and a guide to symptoms of mental health problems among children up to age 3. The guide, by the infant advocacy group Zero to Three, is similar to the popular DSM-IV, a handbook of psychological disorders among older children and adults.

Even without a guide, many psychologists can detect problems in a baby by just looking at him or her, Lieberman says. Stressed-out babies look "sad, withdrawn, frightened and disorganized."

As young as 4 months, mentally ill babies won't smile or laugh, she says, and they may show signs of stress seen in much older people -- digestive problems and weight loss.

As they get older, toddlers who have been exposed to severe stress reveal the after-effects through "post-traumatic play," Lieberman explains. "Their play is rigid and repetitious. They'll repeat the same thing again and again, going over and over it, and there is no emerging. The child cannot get out of it. He's trying, but he cannot."

While experts think they're getting a better handle on diagnosing mental problems facing very young children, treatment remains a challenge. Without the benefit of drugs or psychotherapy, counselors can only change the lives of infants by convincing their caregivers to do things differently.

"You cannot do therapy unless you work with the mother and child, father and child, grandmother and child, or whatever," Honig says.

Dr. Stanley Greenspan, a child psychiatrist at George Washington University, says researchers are teaching parents and other caregivers to adjust how they interact with an infant depending how he or she reacts to sensations. A hypersensitive baby who's sensitive to noise and sound might need extra soothing and comforting, for example.

Therapists must often teach parents to compliment their infants instead of criticize them and make sure the kids feel safe in times of stress, Honig says. In some cases, parents fail the task because they are too focused on their professional lives and can't "unarmor" when they get home, she says.

Whatever the treatment, experts agree that helping infants handle the challenges of life will pay off down the line. "The biggest myth is that it doesn't make a difference what you do in the early years, that people's traits are genetic and you can't have a favorable influence," Greenspan says. "That's not true."

More information

To learn more about infant mental health, visit Zero to Three. You could also try the Colorado Institute for Conflict Resolution and Creative Leadership.