At some point in their lives, a large share of people will struggle with their mental health, and almost everyone will love someone who does. The World Health Organization estimated that in 2019 about one in eight people worldwide, nearly a billion, were living with a mental disorder, with anxiety and depression the most common. This final level looks at how psychology defines and treats mental illness, how understanding has changed, and what research says about living well.

From asylums to communities

For much of history, people with serious mental illness were feared, hidden away or punished. Reformers slowly changed this. In the 1790s the French doctor Philippe Pinel was celebrated for releasing patients from chains in Paris, and in the 1840s the American campaigner Dorothea Dix pushed state governments to build hospitals that would treat patients humanely. Over time, many of those hospitals became overcrowded. In the United States, state hospitals held more than half a million patients in the mid-1950s. New medications and a growing civil rights movement led to deinstitutionalization, as large hospitals closed. The promised network of community care was often underfunded, and many people ended up without support, homeless or in prison.

Defining disorder

How do professionals decide when distress becomes a disorder? Most use the Diagnostic and Statistical Manual of Mental Disorders, or DSM, published by the American Psychiatric Association since 1952, or the World Health Organization's International Classification of Diseases. A pattern of thoughts, feelings or behaviors is usually considered a disorder when it causes significant distress or impairment in daily life, not simply because it is unusual. Diagnosis can help people get treatment and understand their experiences, but categories reflect their time. Homosexuality was listed as a disorder until 1973, when the APA removed it after activism and evidence showed it was not an illness.

A famous 1973 study by David Rosenhan claimed that healthy volunteers faking a single symptom were admitted to psychiatric hospitals and kept for weeks. It shaped decades of debate about diagnosis, though later investigation raised serious doubts about how the study was carried out.

Common conditions

• Depression involves persistent low mood or loss of interest, along with changes in sleep, energy and concentration. The WHO ranks it among the leading causes of disability worldwide.
• Anxiety disorders include generalized anxiety, panic disorder and phobias, in which fear is far out of proportion to real danger.
• Post-traumatic stress disorder can follow terrifying events and involves intrusive memories, avoidance and constant alertness.
• Schizophrenia involves hallucinations, delusions and disorganized thinking. Despite popular belief, it is not "split personality," and most people with mental illness are not violent; they are more often victims of violence than perpetrators.

What helps

Treatment has improved greatly. One of the most researched approaches is cognitive behavioral therapy (CBT), which grew from Aaron Beck's work in the 1960s. CBT helps people notice unhelpful thought patterns, such as "I always fail," test them against evidence and gradually change behavior. It has strong evidence for depression and anxiety disorders. Other therapies, from psychodynamic to family therapy, also help many people, and research suggests that a strong therapeutic alliance, a trusting working relationship between client and therapist, is one of the best predictors of success. Medications such as antidepressants and antipsychotics help many people, often most effectively in combination with therapy. Treatment does not work for everyone, but most people with common mental health problems who receive good care improve.

A major obstacle is stigma, the shame and prejudice that keep people from seeking help. Research shows that personal contact with people who speak openly about their experiences is one of the most effective ways to reduce it, echoing the contact hypothesis from the previous level.

The science of wellbeing

Psychology is not only about problems. Around 2000, Martin Seligman and others called for a positive psychology that studies strengths, meaning and flourishing. One finding stands out above all others: relationships matter. The Harvard Study of Adult Development, which has followed its participants since 1938, has found that warm, close relationships predict health and happiness in later life better than wealth or fame. A 2010 analysis of 148 studies found that people with stronger social ties had about a 50 percent greater likelihood of survival over the study periods. Regular exercise, enough sleep, time in nature and a sense of purpose also support wellbeing.

Worked example: a friend who is struggling

A friend has stopped answering messages, skips plans and says they feel "pointless." What can you do with what this course has taught? Reach out directly and listen without rushing to fix things; feeling connected is protective. Don't assume someone else will act, which is the bystander trap. Gently encourage professional help and offer practical support, such as finding a therapist or going to the first appointment together. If they talk about wanting to die, take it seriously and contact a crisis line or emergency services. And treat them as a whole person, not a diagnosis.

That last point connects psychology to Universal Humanism. Across this course, we have seen that every person's mind is shaped by biology, learning, relationships and circumstance, and that we are all vulnerable to pressure, error and suffering. Psychology at its best does not divide people into the normal and the broken. It shows that the same human nature runs through all of us, and that understanding it, with evidence and compassion, is one of the surest ways to protect the dignity of every person.