Depression is a complex illness. Most cases of depression are caused by a combination of factors, which include genetics, biochemical factors, stress and trauma, medical or physical illness, gender, and background.
Genetics
A person with a parent or sibling with depression is about three times more likely to develop depression than someone without a family history. Depression usually doesn’t develop as a result of genetics alone, but in combination with other factors like stressful life experiences, genetic background may increase an individual’s chance of developing depression.
So far no single gene has been reliably associated with the presence of depression. Like other conditions, such as high blood pressure and diabetes, genetic risk is likely the result of a combination of multiple genes that interact in a way that increases the risk of developing depression. Further research is needed to identify how genes predispose an individual to developing depression, and what steps can be taken to reduce risk for depression in individuals who are genetically predisposed.
Biochemical factors
Our understanding of biochemical factors in depression is still limited. We do know that in most cases of depression, neurotransmitter function is probably disrupted. Neurotransmitters relay information between different parts of the brain and may not function optimally in depressed individuals. There is strong evidence that the serotonin, norepinephrine, and dopamine neurotransmitter systems, also called the monoamine systems, are affected. Many of the medications used to treat depression target these neurotransmitter systems. More recently, interest has focused on other neurotransmitter systems, including glutamate and other biochemicals that interact with these neurotransmitter systems, such as brain derived neurotrophic factor (BDNF).
Stress and trauma
Everyone experiences stress from time to time. For most people, these feelings resolve within days or weeks, and may even contribute to personal growth and resilience. However, stress can sometimes lead to depression, particularly if it is chronic (like ongoing relationship problems or taking care of a loved one with serious illness) or acute and profound, such as sudden, unexpected loss. Sustained or chronic stress, in particular, leads to elevated “stress hormones,” such as cortisol, and reduced serotonin and other neurotransmitters in the brain.
Medical or physical illness
Physical illness can directly affect the brain’s neurotransmitter systems and may increase the risk of developing depression. Physical Illness is often associated with discomfort, confinement, and an inability to participate in enjoyable activities, all of which can negatively impact mood. In turn, depression may increase feelings of pain, social isolation, and hopelessness, which can make it more difficult to cope with other medical conditions.
Certain medical conditions, including heart disease, Parkinson’s disease, multiple sclerosis, and stroke, are associated with rates of depression as high as 40-60%. Our research is exploring the relationship between inflammation, immune function, and depression.
Sex and Gender
Both sex assigned at birth and gender identity may play roles in certain types of depression. About twice as many females as males develop depression overall, although rates of certain forms of depression, such as bipolar depression, are about equal across sexes. Increased risk of depression among females may be due to a number of factors, such as brain biology and hormones. Gender identity and expression may influence how distress is expressed and an individual’s willingness to recognize depressive symptoms and seek treatment, as well. Trans and gender diverse individuals also experience depression at a higher rate than those who are cisgender, which may be due to the increased discrimination and social stigma experienced by these individuals.
Background
Individuals who grow up with relatives suffering from depression are at an increased risk for developing depression later in life. This is likely due to a combination of genetic and environmental risk factors.
Socioeconomic stressors can also affect the prevalence of depression. Differences in wealth/income, education, racial/ethnic minority status, and location can all affect an individual's risk for developing the disorder. Cultural background may also affect how individuals express the symptoms of their depression and the acceptability of particular forms of treatment, such as medication or psychotherapy.