Biological explanations of depression

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Genetics-

Depression does tend to run in families. Gershon et al (1990) found that depression is up to 3x higher in families with a history of the disorder, others have claimed this figure is even higher! 
Weissman (1987) found that family members with a first degree relative with a mood disorder, were 10x more likely to suffer too.

Twin studies into unipolar depression show the following concordanc rates:
MZ twins- 40%
DZ twins- 11%
This suggests a genetic component.

For bi-polar it's even higher:
MZ twins-72% (highest for any psychological disorder)
DZ twins- 14%

However some problems with twin studies include-
Environmental factors
Early studies found it hard to distinguish between MZ and DZ twins.
Even MZ twins may have been reared differently.
Not 100% concordance rate.

Adoption studies-
Wender et al found that the biological parents of adopted children who had developed depression, were 8x times more likely to have the disorder than the adoptive parents. This may show clear evidence for genetics causing depression.

Environmental factors are clearly involved with depression.This could be a negative environment acting on someone who is genetically pre-disposed to depression.
Kendler et al- highest levels of depression in those scoring highly on negative life events and having the genetic pre-disposition.

Specific genes for depression-
Egeland et al- researched 81 members of an Amish community. 4 families within the

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