For the first time, researchers have found that the environment you're raised in is as important as your genes in determining risk for major depression.
In a large retrospective study, researchers looked at depression diagnoses among more than 2.2 million people in Sweden and their parents and found that genetic factors and household environment contributed equally to odds that the illness would be "transmitted" from parents to offspring.
The results - based on comparing adopted and biological offspring from both intact and broken families - contradict many previous findings from twin studies that suggested genetic predisposition plays the larger role in the inheritance of depression, the authors write in JAMA Psychiatry.
"Their sample sizes were much too small and not always representative," said lead author Dr. Kenneth Kendler, a professor of psychiatry and human and molecular genetics at Virginia Commonwealth University in Richmond.
"Adoption studies are probably the most powerful method available to understand the mechanism of parent-offspring transmission," Kendler said by email.
"An important feature of this study was our ability to replicate the results from adoptive and biological parents by findings from step- and not-lived-with parents. This increases considerably our confidence in these findings."
In 2015, almost 7 per cent of all adults in the US, or an estimated 16.1 million individuals age 18 or older, reported having had at least one major depressive episode in the past year, according to the National Institute of Mental Health. The disorder is associated with significant work, school and health problems, substance abuse and an increased risk of death by suicide.
Using data collected from January 1960 through December 2016, Kendler and his colleagues analysed newly available Swedish primary care registries, combined with hospital and psychiatric outpatient records to trace treated major depressive disorder in parents and offspring. They examined five types of families with various combinations of biological or adoptive offspring, intact households, and those with an absent father, a stepfather or both.
The new data indicating that genes are not destiny have a range of implications for research, treatment and child-rearing, experts said.
"There's been this huge move in biological psychiatry to look for the genes involved in mental illness," said Robert Klitzman, a professor of clinical psychiatry and director of the masters of bioethics programme at Columbia University in New York City, who wasn't involved in the study.
"We need to push research money so we're not looking exclusively at genes, but spending sufficient time figuring out the best ways to treat the psychosocial factors of depression," Klitzman said in a telephone interview.
"This study provides strong evidence to support public policy for covering mental health treatment that includes a combination of drugs and therapy."
The study's data highlight that when a mother's mental health suffers, so does her child's, said Dr. Joan Luby of Washington University School of Medicine in St. Louis, who wasn't involved in the research.
"If you have a genetic risk for depression you would be wise to enhance certain elements of the child-rearing environment to protect against transmission of depression," Luby said in a phone interview.
"I personally think it has a lot to do with helping children develop emotionally. They should learn how to experience, process and regulate their emotions as opposed to sweeping them under the rug."
Among the study's limitations, the authors note, is that it is based on registries of people with major depression, which may not represent other groups. Some of the data also are self-reported, meaning it may include "false-negative or false-positive" diagnoses.
Klitzman also pointed out that the data are from Sweden and may not necessarily be similar in the US