One of the biological explanations of depression is genetic factors. To study genetic factors, psychologists study families. Family studies select people who laready have depression and then examine their other family members to see if they have been depressed or might become depressed in the future. If there is a genetic link then the relatives should show hgiher rates of depression then the rest of the population.
Furthermore, there is also research to support the link between genes and depression. There is a mutant gene that starves the brain of serotonin and it has been found to be 10 times more prevalent in depressed patients. Patients with the mutation also failed to respond well to SSRI medications. This suggests that the mutation may underlie a treatment resistant subtype of depression.
Psychologists also study twins to look for a genetic link for depression. McGuffin studied 177 people who already had depression and their twin. The concordance rate was 46% for identical twins and 20% for fraternal twins. This suggests that depression may be inherited. However, for depression to be down to genes completely the concordance rate would need to be 100% but because it's only 46% for identical twins, it means that other factors contribute to depression.
The low genetic concordance rates for depression may be explained in terms of…