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Paris and Michael Jackson: Does Depression Run in Families?
Paris Jackson at a fashion show in Paris, France, on January 21, 2017.
Credit: Pascal Le Segretain/Getty

Paris Jackson recently spoke about struggling with both depression and anxiety. Her father, Michael Jackson, also had depression, she told Rolling Stone, and the younger Jackson was eventually prescribed the same antidepressants that her father had taken.

When family members have the same mental health condition, is it a coincidence, the result of sharing the same household environment or evidence that the condition is heritable? Or is some combination of these things true?

Depression is "in the same class as many other complex disorders, like diabetes," in terms of its heritability, Myrna M. Weissman, a professor of epidemiology and psychiatry at Columbia University in New York, told Live Science. In other words, a person's genetic makeup is thought to play a role in the individual's risk of developing depression, but other factors are at work, too.

Researchers think that the heritability of depression — or the amount of variation in the rate of depression within a population that's due to genetic differences — hovers around 30 to 40 percent, Weissman said. [9 Celebrities Who Spoke Up about Depression]

Depression is a common mental health disorder, affecting an estimated 350 million people worldwide, according to the World Health Organization. The condition usually involves depressed mood, lack of appetite, sleep difficulties and diminished interest in day-to-day activities.

In a 2010 study published in the journal Current Psychiatry Reports, Dr. Falk Lohoff, a psychiatrist at the University of Pennsylvania Perelman School of Medicine, surveyed a vast amount of research on the link between genes and depression. The survey highlighted a few key facts about the connection.

For one, studies involving twins and family members have indicated that mood disorders do have a genetic component, Lohoff said. In addition, studies of family members have shown that first-degree relatives of people with depression are more likely to also have the condition. (A person's first-degree relatives include their parents, siblings and children.)

But researchers are still trying to figure out the reasons behind this finding: If there are genes that increase people's risk of depression, what exactly do those genes do?

In a landmark 1996 study published in the journal Science, researchers found a gene that encodes a protein that transports the chemical serotonin, which helps regulate mood. Many antidepressants target this chemical. The scientists also found that certain variants of this gene may be associated with people's risk of depression. Changes in the gene have been shown to influence the body's ability to transport and take up serotonin, the report said.

However, additional research connecting this gene to depression has produced mixed results, and more studies are needed, Lohoff's 2010 review said. [7 Ways to Recognize Depression in 20-Somethings]

That review concluded that studies have so far yielded little evidence that specific genes determine an individual's risk of depression. The condition, ultimately, is too complex to be neatly broken down into one or two gene associations, and most positive findings have yet to be replicated, the review said. For a mood disorder like depression, a person's environment and lifestyle also contribute to his or her risk, the study concluded.

Life events that have been linked to depression include unemployment, loss of a loved one and psychological trauma, according to the World Health Organization. At times, physical illnesses such as cardiovascular disease can also lead to depression, and vice versa, the organization said.

"It is the sum of inside and outside factors that contributes and influences mental pathology and well-being," Lohoff wrote.

Weissman added that "There is a strong environmental component that triggers an episode, but it is highly familial. That is, it is transmitted across the generations."

In her own research and that of many others, scientists are still trying to better determine the network of genes that are involved, she said.

Originally published on Live Science.