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Suicide is recognised in all human populations, but at widely differing rates. Durkheim who was the first to undertake systematic studies maintained that suicide rates were a function of anomie or lack of coherence in society. More specifically rates are generally increased in those who are mentally ill, particularly with the psychoses (schizophrenia, schizoaffective illness and manic-depressive disorder) and are generally estimated as having a 10% lifetime risk by comparison with approximately 1% in the general population. Rates are also increased in physical disease, particularly when chronic and progressive.
Interestingly evidence from adoption studies suggests a genetic component in predisposition to suicide that is independent of psychiatric illness.
A reduction in suicide rates can be achieved for example by reducing the carbon monoxide content of coal gas, or the availability of certain analgesics.
Most studies show an increase in suicide with age, and an excess in males, but there are exceptions. Some national differences have been related to religion, rates in Catholic countries being generally lower, but such estimates are affected by tendencies to avoid legal registration of a death as a suicide.
Suicide is generally distinguished from attempted suicide or self injury, the age range and sex ratio of the latter being younger and less biased to males respectively.
The dependence of suicide rates on social factors is dramatically illustrated by the phenomena of kamikaze pilots and suicide bombers. the mass suicide of the Jones cult in British Guiana is also salutary.
What is necessary for suicide to be recognised as such is that the individual has the concept of his own life and its discontinuation. Arguably such a concept depends upon the capacity for language.
There are no societies in which suicide is reliably reported to be absent.
A plausible explanation for the apparent difference between Homo sapiens and other species in predisposition to suicide is that only Homo sapiens has the capacity for symbolic representation that allows individuals to develop a concept of their own mortality and in some cases to come to the decision that the circumstances of life outweigh its preservation.
Some cases of suicide are regarded as altruistic according to JBS Haldane's dictum that ' I will jump into the river to save two brothers or eight cousins ', but such logic applies to only a small minority of cases of suicide.
In his review Preti writes ' Naturalists have not identified suicide in nonhuman species in field situations, despite intensive study of thousands of animals species', and ' Sparse evidence supports some resemblance between the self-endangering behaviour observed in the animal kingdom, particularly in animals held in captivity or put under pressure by environmental challenges, and suicidal behaviour among humans '.