Depression in children as young as three is real and not just a passing grumpy mood, according to provocative new research.
The study is billed as the first to show that major depression can be chronic even in very young children, contrary to the stereotype of the happy-go-lucky preschooler.
Until fairly recently, "people really haven't paid much attention to depressive disorders in children under the age of six," said lead author Dr Joan Luby, a psychiatrist at Washington University in St Louis.
"They didn't think it could happen ... because children under six were too emotionally immature to experience it."
Previous research suggested that depression affected about 2% of United States preschoolers, or about 160,000 youngsters, at one time or another.
But it was unclear whether depression in preschoolers could be chronic, as it can be in older children and adults.
Dr Luby's research team followed more than 200 preschoolers, aged three to six, for up to two years.
The group included 75 youngsters diagnosed with major depression.
The children had up to four mental health exams during the study.
Among initially depressed children, 64% were still depressed or had a recurrent episode of depression six months later, and 40% still had problems after two years.
Overall, nearly 20% had persistent or recurrent depression at all four exams.
Depression was most common in children whose mothers were also depressed or had other mood disorders, and among those who had experienced a traumatic event, such as the death of a parent or physical or sexual abuse.
The study, funded by the National Institute of Mental Health, did not examine depression treatment, which is highly controversial among children so young.
Some advocates say parents and doctors are too quick to give children powerful psychiatric drugs.
Though sure to raise eyebrows among lay people, the notion that children so young can be depressed is increasingly accepted in psychiatry.
University of Chicago psychiatrist Dr Sharon Hirsch said the public thought of preschoolers as carefree.
"They get to play. Why would they be depressed?" she said.
But depression involved chemical changes in the brain that could affect even youngsters with an otherwise happy life, said Dr Hirsch, who was not involved in the study.
"When you have that problem, you just don't have that ability to feel good," she said.
And, in fact, Dr Luby said she had separate, unpublished research showing that chemical changes seen in older children also occurred in depressed preschoolers.
Dr Helen Egger, a Duke University psychiatrist who has studied childhood depression, said it was common in her field to first see depressed children in their teens.
Their parents will say symptoms began very early in childhood.