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Avoidance of inbreeding

Although the main adaptive function of human sexual activity is reproduction, human sexual activity also includes the adaptive constraint of avoiding close inbreeding, since inbreeding can have deleterious effects on progeny. Charles Darwin, who was married to his first cousin Emma Wedgwood, considered that the ill health that plagued his family was a consequence of inbreeding. In general, inbreeding between individuals who are closely genetically related leads to the expression of deleterious recessive mutations. The avoidance of inbreeding as a constraint on human sexual activity is apparent in the near universal cultural inhibitions in human societies of sexual activity between closely related individuals. Human outcrossing sexual activity provides the adaptive benefit of the masking of expression of deleterious recessive mutations.