What the genetic explanation claims

1. The genetic explanation says that part of the risk of developing a psychological problem is inherited, passed from parents to children in their genes.
2. Genes influence how the brain develops and how its chemical messengers work, so they can shape how a person responds to difficult experiences.
3. What is inherited is a genetic vulnerability, meaning a raised risk rather than a certainty, so the same genes can lead to very different outcomes in different lives.

How psychologists look for a genetic influence

1. Psychologists look first at whether a problem runs in families, because a genetic explanation predicts that close relatives will be affected more often than unrelated people.
2. Twin studies sharpen this by comparing identical twins, who share almost all their genes, with non-identical twins, who share about half.
   1. A higher concordance rate for identical pairs points to genes rather than upbringing, because both kinds of twin usually grow up in the same home.
3. Concordance is never complete, and that gap is itself evidence that genes cannot be acting alone.

Genes and depression

1. Depression tends to run in families, which is the pattern a genetic explanation predicts.
2. Caspi et al. (2003) genotyped 847 members of the Dunedin longitudinal cohort for a variation in the 5-HTTLPR gene, which affects serotonin, a brain chemical linked to mood.
   1. The three groups were 147 people (17 per cent) with two short alleles, 435 (51 per cent) with one short and one long, and 265 (31 per cent) with two long.
3. The researchers recorded 14 stressful life events falling between participants' 21st and 26th birthdays and assessed depression at 26 using the Diagnostic Interview Schedule together with an informant questionnaire.
4. The number of stressful events did not differ between the genotype groups, so the gene did not change how much stress people met.
5. Carriers of a short allele showed a significantly stronger link between life events and depression, and childhood maltreatment predicted adult depression only in short-allele carriers.

Why the gene alone is not the cause

1. What Caspi et al. found is a gene by environment interaction, better known as diathesis-stress, in which an inherited vulnerability only leads to depression when stressful experience acts on it.
   1. Two people can carry the same short allele and only the one who meets serious life stress becomes depressed.

Strengths and weaknesses for depression

1. One strength is the quality of the evidence, since Caspi et al. followed a large cohort over many years, used real clinical diagnoses and added informant reports, so the findings do not rest on self-report alone.
2. A second strength is explanatory reach, because the diathesis-stress version accounts for something a purely environmental account cannot, namely why two people meeting the same losses do not both become depressed.
3. One weakness is that the design is correlational, so it cannot establish that the short allele causes depression, and later attempts to replicate the interaction gave mixed results.
4. A further weakness is that the explanation is reductionist, reducing depression to genes and leaving out the patterns of thinking that cognitive theory describes.

Genes and addiction

1. The same logic is applied to addiction, so what is inherited is a raised susceptibility rather than the addiction itself.
2. The vulnerability is thought to lie in inherited differences in brain chemistry, because genes affect how neurotransmitters work at the synapse.
   1. If those inherited differences make the effect of a substance or an activity feel stronger for one person than for another, they are more likely to repeat it and to keep repeating it.
3. Addiction also runs in families, and twin studies are used here in the same way as for depression, comparing identical with non-identical pairs.
4. A vulnerability can only show itself if the person actually meets the substance or activity, so opportunity and environment stay part of the explanation.

Strengths and weaknesses for addiction

1. One strength is that it explains individual differences well, since many people use a substance and only some go on to develop an addiction.
2. A second strength is that it can be tested with objective family and genetic data rather than relying on what people say about their own habits, which reduces the risk of biased self-report.
3. One weakness is that families share their surroundings as well as their genes, so a family pattern could reflect shared habits, stress or availability rather than inheritance.
4. A further weakness is that it is reductionist, leaving out how use is reinforced and how it is copied from other people, which is what learning theory describes.

Turning this into marks

1. Check whether you are being asked about depression or about addiction and keep your evidence to that half.
2. A bare claim that depression is genetic scores nothing until you add the mechanism behind it.
3. The mark most often lost is quoting Caspi et al. without stating the point it proves, which is that the gene interacts with life stress.