Health experts’ understanding of depression has come a long way in the last few decades. In many cases, depression doesn’t have only one cause. It often results from a mix of biology, psychology, and stressful or traumatic events.

Reasons for depression include:

1. Biology. Researchers still have a lot to learn about exactly why people become depressed. But some experts say depression often results from an imbalance of certain chemicals in the brain. These chemicals are called neurotransmitters. Antidepressants work by affecting the levels of these chemicals — bringing them back into balance.

There’s also a genetic connection. If depression runs in your family, you have a higher chance of becoming depressed.

Having other general health conditions or diseases can increase your risk of depression. For instance, if you have a heart attack, you have a 65% risk of becoming depressed afterwards. Conditions such as cancer, heart disease, thyroid problems, and many others increase your risk of becoming depressed.

Your gender makes a difference, too. Women are about twice as likely as men to become depressed. No one’s sure why. But the hormonal changes that women go through at different times of their lives may be a factor.

2. Psychology. Studies show that people who have a pessimistic personality are more likely to become depressed. This doesn’t mean that getting a “better attitude” will resolve your depression.

3. Stressful events. Many people become depressed during difficult times. Losing a family member or close friend, being diagnosed with a serious illness, going through a divorce, or any other traumatic event may trigger depression.

4. Medications and substances. Many prescription drugs can cause the symptoms of depression. Alcohol or substance abuse is common in depressed people. It often makes their condition worse.

Some people have a clear sense of why they become depressed. Others don’t. The most important thing to remember is that depression is not your fault. It’s not a flaw in your character. It’s a disease that can affect anyone.