2
Theories and Models of the Disorder
2.1 Biological Models
Historically, depression has always been seen as, at least in part, somatic or biological in origin. Modern genetic studies, biochemical analyses, and scanning methods support this position. Diagnostic distinctions have been made on the basis of assumed primary biologic (endogenous) versus primarily environmentally driven (exogenous) depressions. Modern theories of the nature of the biological contribution have evolved over the last few decades as investigative research techniques have evolved.
2.1.1 Genetics
Genetic studies indicate that depression has a genetic component that interacts with the environment to produce the disorder
The degree to which genetics contribute to the occurrence of a disorder is studied by three primarily methodologies. Family studies start with a patient proband and identify the percentage of relatives of varying degree who also have the disorder. The studies are relatively easy to do, but do not separate the effects of genetics from the effects of shared environments. Twin studies compare the co-twins of proband twins who have the disorder. Monozygotic co-twins who share 100% of genetic material should be twice as likely to have the disorder as dizygotic co-twins who share 50% of genetic material. In these studies, environments are relatively constant, so a purer assessment of genetics is possible. In adoption studies, the children of ill parents who are adopted at a young age are compared to children who grow up with their biological parents. If they have the same elevated rates of the disorder, then genetics rather than environment is assumed to be responsible.
All three methodologies support the presence of a genetic component to depression. The precise estimates of the accounted-for variance vary considerably, but it is generally agreed that genetics are a significant, but not substantial, contributor that accounts for less than 50% of occurrence. Shared environment seems to play a relatively small role, whereas individual life events contribute significantly.The genetic contribution is greater for more severe, recurrent, melancholic, or psychotic depressions. A major role of genetics appears to be to increase the impact of negative life events in precipitating episodes of depression. Genetics provide the biological diathesis or vulnerability that interacts with environmental stress.
Studies comparing the heritability of bipolar versus unipolar depression find thatthe genetic component of bipolar is greater than that for unipolar disorder. In the families of bipolar probands, both bipolar and unipolar relatives are common. In contrast, primarily unipolar relatives are found in the families of unipolar probands. It is generally agreed that both disorders involve multiple genes, i.e., neither is due to a single dominant or recessive gene. The difference between bipolar and unipolar families has been taken to mean that the two disorders share some genes, but it may require a greater concentration of genes to produce bipolar disorder. Thus, in bipolar families you find more ill relatives, some with h