"Adolescent depression can be as high as 20 percent or higher," said Hammen, who has studied depression for more than 30 years. "While these numbers are alarming, and depression can be impairing, most of the depressions will be short-lived and most of these kids will not go on to have further depression.
"Many things can make teenagers depressed, but in about 60 percent of the cases, the depression does not foretell future depression. Of those who have adolescent depression, perhaps 40 percent will have recurring depression, and many of those will likely be life-long."
Hammen has been conducting a long-term study over the last 10 years of 800 families with children now in their 20s, along with Patricia Brennan from Emory University in Atlanta. The subjects are among 7,000 families in Australia who have been studied since the children were five years old by researchers from the University of Queensland.
About half of the children in the current study who showed depression by age 15 had recurring depression by age 20, and those who did have a distinctive profile, Hammen and Brennan found. Their preliminary analysis indicates the adolescents who showed depression by age 15 and again between 15 and 20 also had anxiety disorders and poor social relationships such as fewer friends or more conflict in their relationships.
"We have found that the risk for recurring depression occurs in the kids who had early depression and social difficulties," Hammen said. "Kids depressed by 15 who function normally in social relationships did not go on to have depression by 20. Poor social functioning by age 15, such as not having stable friendships, looks like a risk factor for recurring depression.
"The kids with depression at both 15 and 20 have a very high rate before age 15 of fears, phobias, anxiety, and panic reactions," she said. "In many cases, they were showing anxious, fearful and distressed symptoms as early as age five."
Twenty percent of the children of depressed mothers developed depression by age 15, a rate of depression twice as high as that of the children of non-depressed mothers.
Girls were more than twice as likely as boys to show depression at both ages 15 and 20, Hammen said.
Parents commonly do not see depression in their children, she said.
"Parents often don't pick up on inner despair or distress, and notice depression mainly as irritability or loss of enjoyment of activities their kids used to enjoy," Hammen said. "If it goes on for a month, I think the parent should pay more attention and not just dismiss the behavior as part of adolescence."
"Most of the time when children are depressed, there's something wrong in their lives, and the situation that's causing the depression needs to be addressed," Hammen said. "Kids often get depressed because bad things happen to them that they don't have the coping skills to deal with."
Women are especially at increased risk for depression from stress factors, particularly interpersonal stress, such as poor relationships marked by as conflicts.
"Most depression, particularly for women, is triggered by negative social connections, such as relationship problems, arguments, break-ups, and not having close friends," Hammen said.
Interpersonal stress plays a large role in women's depression, Hammen has found.
Not all depression is alike, Hammen said, noting it can be mild or severe, a single episode or recurring.
Major episodes of depression last for at least two weeks, with impairment in performing daily activities, and a combination of symptoms such as a depressed mood, loss of enjoyment in previously pleasurable activities, changes in appetite, changes in sleep patterns, loss of motivation and energy, difficulty concentrating, and negative thoughts about themselves and the future, including suicidal thoughts.
People who are depressed often select themselves into stressful environments, or contribute to such environments through difficult relationships with romantic partners or families, that may result in chronic or recurring depression for themselves, Hammen said.
Hammen has studied depression in adolescents and adults, and has studied children of depressed mothers. Depressed mothers often themselves had mothers who were depressed. Her research is funded by the National Institute of Mental Health.
"By age 15, the children of depressed mothers had a higher risk of depression, but the risk for depression is not just from genetics," Hammen said. "The risk is from a cascade of events that occur in depressed families, environmental and psychological factors that are the determinants of whether the kids become depressed.
"If the mother is depressed and had stressful family factors like a bad marriage, then the child is more likely to have social and interpersonal problems and more likely to have stressful life events associated with depression."
In the Australia study, Hammen is testing whether it makes a difference when in the child's life the mother was depressed and whether the mother's depression was severe or mild. Mild depression does not have much impact on the child unless it is chronic, she said.
Hammen and Brennan plan to study the link between maternal depression and youth depression, as well as interpersonal problems such as domestic violence and early pregnancy.
Hammen has debunked myths about depression, including this one:
"A lot of people think depression stems from a weakness in character, and that you should just keep a stiff upper lip," Hammen said. "It's a myth. People cannot simply will depression away. It can be a serious impairment, and may require treatment. It's not a weakness of will."