No single cause
People often want one explanation: a chemical imbalance, a bad year, a difficult childhood. The research points to something messier. The National Institute of Mental Health (NIMH) notes that genetic, biological, environmental and psychological factors all play a role. The NHS puts it simply: there is no single cause of depression, it has many different triggers, and different causes often combine.
That is not a dodge. It is useful. If depression comes from several sources, then there are several places where change can start. A person whose low mood began after a layoff, in a family with a history of depression, during a winter of poor sleep and more drinking, has more than one thread to work with, and pulling on any of them may help.
Genes and family history
Depression can run in families. The NHS notes that if a parent, sister or brother has had depression, you are more likely to develop it too. The American Psychiatric Association cites twin research: if one identical twin has depression, the other has a 70 percent chance of having it at some point in life.
Genes are not destiny. The NHS adds that depression is usually caused by a combination of triggers, so a family history does not mean you will develop it. Even the twin figure shows that people with identical genes do not always share the illness. Knowing your family history is mainly useful as information for you and your clinicians.
Brain chemistry and biology
Both the American Psychiatric Association and Cleveland Clinic describe differences in brain chemicals such as serotonin and dopamine as possible contributors. You may have heard depression described as a chemical imbalance. That phrase captures part of the picture, but biology interacts with everything else on this page: stress affects the body, and the body affects mood.
Hormonal transitions matter for some people. The NHS mentions pregnancy and birth and the first years of menopause as times when depression can be triggered. Perimenopause, Menopause and Changes in Mood covers that in detail.
Physical health and medications
Cleveland Clinic lists several conditions associated with higher rates of depression, including cancer, chronic pain, diabetes, multiple sclerosis, Parkinson's and Alzheimer's disease, seizure disorders and stroke. NIMH adds heart disease and notes that depression and these illnesses can make each other worse. It also notes that medications taken for an illness can sometimes contribute to depression symptoms.
Some conditions can produce symptoms that look like depression. NIMH names thyroid disorders and viruses, and the NHS notes that an underactive thyroid can cause depression in some people and that head injuries are an often under-recognized cause. This is why a medical checkup belongs early in any evaluation. Medical Conditions and Medications That Can Cause Anxiety or Depression goes further. If you suspect a medication, talk with your prescriber before changing anything.
Stress and life events
Painful events are among the most familiar triggers. The NHS lists bereavement, divorce, illness, losing a job, and job or money worries. Cleveland Clinic includes loss, conflict and lack of support. The American Psychiatric Association adds continuous exposure to violence, neglect, abuse or poverty as environmental risks.
The NHS describes how events can stack into what people call a downward spiral. A relationship ends, you feel low, you stop seeing friends, you start drinking more, and each step makes the next one more likely. It also notes that the risk of becoming depressed after a stressful event rises when people stop seeing friends and family and try to handle problems on their own.
Not every hard period becomes depression. Depression vs. a Difficult Season looks at how the two differ.
Personality and thinking patterns
The American Psychiatric Association notes that people with low self-esteem, who are easily overwhelmed by stress, or who are generally pessimistic may be more likely to experience depression. The NHS mentions low self-esteem and being overly self-critical, and suggests these traits may come from genes, early life experiences or both.
These are not character flaws. Patterns like harsh self-criticism usually develop for understandable reasons, often early in life, and they can change. Exploring where they came from and how they show up now is a central part of insight-oriented therapy.
Loneliness, alcohol and drugs
The NHS lists loneliness, such as being cut off from family and friends, as something that can increase the risk of depression. It also notes that drinking too much or using drugs to cope can lead to a spiral of depression, and that alcohol affects the chemistry of the brain in ways that increase risk. Loneliness, Isolation and Depression explores the first of these.
Why the cause matters for treatment
Clinicians rarely need to find the one true cause. What helps is a working picture of your particular mix: what was going on when it started, what keeps it going, and what has helped before. The American Psychiatric Association describes an evaluation that covers symptoms along with personal, medical and family history, plus a physical exam to screen for conditions that can mimic depression.
Treatment usually involves psychotherapy, medication or both, according to NIMH. Dr. Lehmann's depression therapy is insight-oriented and integrative: it attends to history, relationships and patterns as well as practical coping for the week ahead. She sees adults in Franklin and by telehealth for clients in Michigan, and does not prescribe.
It is not your fault
Whatever combination of factors is involved, depression is a health condition, not a sign of weakness or a lack of effort. NIMH notes that it can affect anyone, regardless of age, sex, race or ethnicity, income, culture or education.