Biological explanation of depression
IB Psychology· Health and well-being· 14 min read
1. Core concept
A biological explanation says that a mental health disorder like depression has physical roots — in genes and in brain chemistry. Twin and family studies suggest depression runs in families (a heritability of roughly a third), and reduced serotonin-system function is linked to low mood, which is why many antidepressants target it. Crucially, genes and chemistry set a vulnerability rather than a fixed fate: they raise risk, and whether depression appears usually depends on that vulnerability meeting life stress (the diathesis-stress idea). So a biological explanation is one probabilistic part of the picture, not the whole cause.
2. Key studies
IB Psychology answers must be supported by named studies. Learn these to cite as evidence:
Sullivan, Neale, and Kendler (2000)
A meta-analysis of twin studies estimated the heritability of depression at about 37%. — Quantifies the genetic contribution to depression — and, by leaving ~63% to other factors, shows genes are only part of the cause.
Caspi et al. (2003)
People with the short 5-HTT (serotonin-transporter) allele showed more depressive symptoms after stressful life events, while the genotype alone did not predict depression. — Anchors the genetic/neurochemical explanation while showing it works only in interaction with stress — biology as vulnerability, not destiny.
3. Evaluation (AO3)
Strength — measurable and treatable — Biological factors can be estimated (twin designs, genotyping) and give testable, treatable targets — for example drugs acting on serotonin.
Limitation — single-factor and correlational — Heritability is a population statistic and the gene-depression link is largely correlational; a lone gene or 'chemical imbalance' cannot on its own explain a disorder also shaped by cognition and environment.
4. Scope
5. Common Pitfalls
Wrong move:
Depression is simply caused by a 'chemical imbalance' (too little serotonin) or by a single 'depression gene'.
Why:
A frequently penalised misconception in IB Psychology exams.
Correct move:
Biological explanations describe a vulnerability that raises risk and interacts with the environment — heritability is only ~37%, and Caspi shows the gene predicts depression only alongside stress. Present it as one interacting factor, not the sole cause.
Going deeper
- Genetic inheritancethe heritability estimate used here is genetic inheritance (BIO_6).
- Diathesis–stress modelthe gene x stress interaction is the diathesis-stress model (BIO_5).
- Biological treatment (depression)the biological treatment (SSRIs) targets this explanation (HWB_8).