Depression is a complex mental health condition influenced by multiple interacting factors, not a single cause. Experts classify these into biological, psychological, social, and environmental categories.

Biological causes

• Brain chemistry imbalance: Low levels of neurotransmitters like serotonin, norepinephrine, and dopamine disrupt mood regulation.
• Genetics: Having a first-degree relative (parent/sibling) with depression increases risk 2–3 times (NIMH data). Specific gene variants affect stress response.
• Hormonal changes: Pregnancy, postpartum period, menopause, thyroid disorders, or cortisol dysregulation (chronic stress) can trigger episodes.
• Medical conditions: Chronic illnesses (diabetes, cancer, Parkinson’s, heart disease) and certain medications (beta-blockers, steroids) raise risk.

Psychological causes

• Negative thinking patterns: Persistent self-criticism, perfectionism, or rumination keeps the brain in a low-mood loop.
• Trauma & adverse childhood experiences (ACEs): Physical/sexual abuse, neglect, or parental loss before age 18 significantly raises lifetime risk.
• Personality traits: Low self-esteem or high neuroticism makes individuals more vulnerable.

Social & environmental causes

• Stressful life events: Job loss, divorce, financial crisis, bereavement, or caregiving burden can precipitate episodes.
• Social isolation: Lack of supportive relationships doubles depression risk (WHO).
• Poverty & discrimination: Ongoing socioeconomic disadvantage, racism, or marginalization creates chronic stress.
• Substance use: Alcohol or drug dependence both causes and worsens depression; 30–40% of people with substance-use disorders have co-occurring depression.

Lifestyle factors Poor sleep, sedentary behavior, unhealthy diet, and lack of sunlight (seasonal affective disorder) further contribute.