Explaining Depression: Genes and Cognitive Theory
Two Explanations for Depression
Depression does not have one single cause. This lesson covers two explanations you need to know: a biological explanation based on genes, and a psychological explanation called cognitive theory — the idea that patterns of thinking cause and maintain low mood. These explanations are not rivals: many psychologists now see depression as arising from an interaction between genetic vulnerability and the way a person thinks and interprets events.
This lesson covers two explanations of depression: the genetic explanation (depression is inherited through genes, e.g. the 5-HTT gene) and cognitive theory (depression is caused and maintained by negative patterns of thinking).
The Genetic Explanation
The genetic explanation proposes that some people inherit a biological vulnerability to depression from their parents. Family and twin studies show depression tends to run in families, and identical twins (who share all their genes) are more likely to both experience depression than non-identical twins (who share only some genes). This pattern suggests genes play a role in who develops depression, although it does not prove genes are the only cause.
Researchers have looked for specific genes linked to depression. One focus is the gene controlling the serotonin transporter, involved in regulating mood. This gene shows a polymorphism — it exists in a short form and a long form. Carrying the short form has been linked to greater depression risk, but research suggests this gene does not cause depression on its own. Instead it appears to interact with a person’s environment, an idea called gene-environment interaction. A person may carry the higher-risk gene their whole life without ever developing depression, unless they also experience significant environmental stress.
Strength: family and twin study evidence, and gene research such as Caspi et al. (2003), gives objective, scientific support that is hard to dismiss as coincidence. It also explains why depression can appear in families with no obvious environmental cause.
Weakness: the genetic explanation is reductionist — it risks reducing a complex human experience to biology alone. No single “depression gene” has been found, and many people with the higher-risk gene never develop depression, showing genes cannot be the whole explanation.
Cognitive Theory of Depression
Cognitive theory explains depression as a product of a person’s own thinking, not their biology. This approach, associated with the psychologist Aaron Beck, argues depressed individuals develop negative schemas — fixed, pessimistic patterns of thinking about themselves, the world and the future. Once formed, these schemas act like a filter: a person notices and remembers information that fits their negative view, and dismisses information that contradicts it.
Cognitive theory also focuses on attributions — the explanations people give for events in their lives. Someone vulnerable to depression tends to attribute negative events to internal causes (“it’s my fault”), stable causes (“this will always happen”) and global causes (“this ruins everything”). This attribution style makes setbacks feel permanent and inescapable, deepening low mood. Someone with a more resilient thinking style attributes the same setback to external, unstable and specific causes and recovers more quickly.
Cognitive theory explains depression through negative schemas and a negative attribution style: blaming negative events on internal, stable and global causes makes setbacks feel permanent and unavoidable, maintaining low mood.
Strength: cognitive theory has led directly to effective talking therapies, such as cognitive behavioural therapy, which help people identify and challenge negative thinking patterns. This practical success is often used as evidence that thinking patterns really do play a causal role in depression.
Weakness: it is difficult to know whether negative thinking causes depression or whether depression causes negative thinking — a cause-and-effect problem. The theory may describe a symptom of depression rather than its origin.
Caspi et al. (2003): Life Stress and the 5-HTT Gene
Aim: Caspi et al. (2003) aimed to investigate whether a polymorphism in the 5-HTT gene moderates the relationship between stressful life events and the onset of depression. Procedure: the researchers studied a large group of young adults from a long-running study whose genotype for the 5-HTT gene was already known. Participants were grouped according to whether they carried the short or long version of the gene. Researchers then measured how many stressful life events each participant had experienced over a set period, and assessed each participant for symptoms of depression.
Results: participants who carried one or two copies of the short version of the 5-HTT gene showed more depressive symptoms, a higher likelihood of a diagnosable depressive episode, and greater suicidality in response to stressful life events than participants who carried only the long version. Critically, the gene on its own did not predict who became depressed — its effect only appeared in combination with stressful life events.
Conclusion: Caspi et al. concluded that genes do not cause depression in isolation. Instead, a genetic vulnerability (the short allele of the 5-HTT gene) increases sensitivity to stress, and it is the combination of this vulnerability with environmental stress that raises the risk of depression. This supports the gene-environment interaction model rather than a purely genetic explanation.
Strength: the study used a large sample followed over time, which increases generalisability, and genotyping is an objective, scientific measure that improves the reliability of the gene classification.
Weakness: the study is correlational — it identifies a relationship between the gene, stress and depression, but it cannot prove the gene causes depression. Self-reported life stress may also be subjective, reducing the validity of that measure.
Genes and Cognitive Theory: A Combined Picture
Rather than competing, these two explanations can work together. A person may inherit a genetic vulnerability, such as the short allele of the 5-HTT gene, which makes them more sensitive to stress. If that person then develops negative schemas and a negative attribution style — perhaps shaped by difficult early experiences — the combination of biological vulnerability and negative thinking can make depression far more likely than either factor alone.