Kin support may be a key difference between the EEA and contemporary society and so may account for depression and other psychological disorders. The family and community do not form close-knit bonds anymore, as they did in our evolutionary past.
However, according to Crawford the differences between the EEA and modern environment have been exaggerated because they ignore the fundamental similarities of human existence then and now, i.e. we are social beings who form relationships, have children, gossip, compete for resources and position. This does not invalidate genome lag but suggests we need to focus more clearly on the specific differences that lead to psychological disorders.
The second general evolutionary explanation of mental disorders is based on ‘increased fitness’. This explanation assumes that the genes underlying certain mental disorders are also responsible for other more desirable traits that aid our survival and so are a kind of side effect of other adaptive traits.
According to Nesse and Williams it may be useful to be depressed as the apathy and withdrawal associated with this may aid survival. For example, the impulse to hide under the bed covers may stem from our evolutionary ancestors hiding in caves, which could increase survival in times of bad weather.
A similar theory that depression is adaptive has been proposed by Allen. Human social groups are hierarchical and so conflicts arise over position and authority. According to Allen’s rank theory of depression this emotional response is adaptive when the individual has lost because accepting loss and the consequent withdrawal ends the conflict. The loser retreats and so is protected from further injury and the winner has a clear sense of victory.
For an evolutionary explanation of depression to have any validity there must be evidence of a genetic base to the disorder and the illness must exist across al cultures. There is evidence of a genetic basis to unipolar depression as shown by twin studies where concordance rates of between 40% and 59% have been found, and adoptive studies which show that depression is common to biological relatives but not adoptive relatives. Depression is a relatively universal condition, as it exits across cultures which further supports a genetic basis.
There is evidence against this theory. Disorders may not be weeded out because they are recessive, i.e. a person may carry the gene but not exhibit the disorder, which only manifests when both parents carry recessive alleles. Thus, it is naturally selected not because of increased fitness but because it is difficult to eliminate as recessive genes do not affect the individual’s reproductive fitness. Another contradiction to the evolutionary explanations of adaptiveness is gene – mapping studies. Sometimes genes can change and mutate. This change in structure to the gene could cause unipolar depression.
Evolutionary explanations of bipolar depression in terms of increased fitness, show that there is a much stronger evidence of a genetic basis to bipolar then unipolar depression as the concordance rate for bipolar in MZ twins is 80% compared to 16% in DZ.
There is also evidence that a relationship seems to exist between bipolar depression and creativity and/or leadership. Creativity and leadership would be adaptive traits to have in the past as the creative person would be able to find novel solutions to problems and leadership quality may create a harmonious group.
Evaluation of evolutionary theories of mental disorders would start with conjecture. Deciding what was dangerous in our evolutionary past is highly speculative. Such explanations are post hoc and do have little scientific basis.
This theory is also deterministic and reductionist, as they suggest that the genes control behaviour, which ignores the free will of the individual and also because they focus on one factor only, the gene, when other factors such as emotion, society and behaviour are highly relevant to the aetiology of mental disorders.
Evolutionary explanations need to be considered in combination with other explanations. Both biological and psychological factors interact in the aetiology of mental disorders as the compromise position of the diathesis-stress model suggest.
