There isn’t usually one clear cause of depression. Instead, major depressive disorder and other types of depression often develop from a mix of factors. These risk factors can make someone more likely to develop depression, but they don’t guarantee that they will.

According to recent data, 8.3 percent of adults in the United States had a major depressive episode over the course of one year. In this article, we’ll take a look at what causes depression, as well as factors that can increase your risk of developing it.

Genetics and Family History

Depression can run in families. This means your genes may play a role in your risk.

People who have a first-degree relative, such as a parent or sibling, with depression or another mental health condition like bipolar disorder, may be more likely to develop depression.

Some research suggests that genes passed down in families account for about half of depression risk. In other words, certain genes can increase your risk of developing depression, but they don’t guarantee you will. There isn’t a single “depression gene” that determines whether you’ll develop it.

Instead, many genes may contribute to your risk. A recent study found that 80 distinct genetic areas were linked to major depression. Twelve areas were linked to early-onset depression. These genes can interact with your life experiences, environment, and other factors.

Environmental and Life Experiences

Outside factors may also increase your risk of depression. People who are more likely to develop depression due to their genes may find that certain events or situations trigger it.

Seasonal Changes and Light Exposure

For some people, changes in daylight may play a role in depression. Seasonal affective disorder (SAD), for example, is a type of depression most often linked to less natural light during the fall and winter.

SAD symptoms usually go away as the days become longer and sunnier in spring and summer. Less often, SAD follows the opposite pattern and develops in the spring or summer.

Researchers think that winter-onset SAD may be caused, in part, by how reduced sunlight affects the hypothalamus. That’s the part of the brain that controls your internal clock. This area of the brain is sensitive to light and also helps manage sleep, mood, and other important functions.

Lack of sunlight in winter-onset SAD may also impact hormone levels. Melatonin, which helps manage sleep and mood, may become unbalanced. Levels of serotonin — a chemical in the brain linked to mood — may also fall, which can play a part in triggering depression.

Stress and Trauma

Difficult or stressful life events can raise the risk of depression. This is especially true in people who are already more likely to develop it. For those at higher risk, the loss of a loved one, violence, financial strain, accidents, or other big challenges may trigger depression.

Studies have shown that childhood trauma can have a lasting effect on mental health. Those who experience abuse, neglect, or other traumatic events as kids may be more likely to develop depression later in life.

Research also suggests that early life stress may alter how the body responds to stress. It may also cause changes in brain function, emotion management, and inflammation.

These changes may increase the risk of depression in people who are more vulnerable. This is particularly true if they experience more stress later in life.

Social Isolation

Social isolation can occur after a major life change, such as a breakup, loss, or move. For many people, disconnection from family, friends, or community contributes to depression.

One study found that people who were more socially isolated had a higher chance of having depressive symptoms. But it also noted that people with more depressive symptoms were more likely to become socially isolated. These findings suggest that isolation, loneliness, and depression may make each other worse.

Pregnancy and the Postpartum Period

For some people, physical, hormonal, and emotional changes during pregnancy and after childbirth may cause depression. You may hear this type of depression called perinatal or postpartum depression. These terms describe the time frame in which symptoms start.

The perinatal time frame includes the time when a woman becomes pregnant and lasts until one year after the baby is born. The postpartum period includes the time after childbirth and lasts until one year after the baby is born.

Hormonal changes during and after pregnancy may play a role in perinatal or postpartum depression. But they’re not the only factors involved. Several factors can increase your risk for these types of depression, including:

• Having had depression in the past or having a family history of depression
• Living with another mental health condition, like bipolar disorder
• Experiencing breastfeeding difficulties
• Having limited social support to care for your baby
• Dealing with financial or relationship stress
• Caring for a baby or babies who need extra care
• Encountering pregnancy or childbirth complications

The Bottom Line

Depression usually develops from a mix of factors rather than one clear cause. Genetic risk, brain chemistry, life experiences, and health conditions can combine in ways that increase the risk of depression.

The way these factors interact may help explain why depression affects people differently. It may also shed light on why certain depression treatments work better for some people than for others.