In one line: No single cause explains depression: genes, thinking and circumstances each add part of the risk.
Explaining a disorder means asking why this person, and why now? Each approach answers part of that question; the biopsychosocial model puts the parts together.
This page is about the biological part of the answer: genes and brain chemistry.
What HL adds: The culture extension: what counts as a symptom differs between cultures.
Paper 3 sources on health often use interviews: ask how the researcher's own view of illness shaped the findings.
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Key idea: Twin studies estimate how much of the risk of depression comes from genes. The answer: some, but far from all.
Reading a twin study
Identical (MZ) twins
Share all their genes. If genes matter, both twins should often be depressed.
Non-identical (DZ) twins
Share about half their genes, like other brothers and sisters.
The gap
The more often identical pairs match than non-identical pairs, the bigger the role of genes. This gives the heritability.
Identical · Non-identical · The gap
Kendler and colleagues (2006) used the Swedish national twin register, about 42,000 twins. The heritability of major depression was about 38%: higher in women (about 42%) than in men (about 29%).
What HL adds: Paper 3 sources may give twin data as a table. Compare the identical and non-identical match rates, then say what the gap cannot show: shared homes also make twins alike.
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Key idea: Longitudinal studies show what comes first; none shows a single cause.
What supports it
- Alloy: thinking style came before depression
- Twin and family studies: genes add risk
- Christakis and Fowler: social ties predict weight gain
Where to be careful
- Correlational designs cannot prove cause
- Symptoms are counted differently across cultures
- One-cause answers are reductionist
So the evidence supports several interacting explanations, and the biopsychosocial model is the strongest conclusion, but most studies show links, not causes.
Watch out: 'Comes before' is not 'causes'. Alloy's study is still correlational.