The evolutionary approach, based on the theories of Charles Darwin, attempts to explain age disparity in sexual relationships in terms of natural selection and sexual selection.[31][32] Within sexual selection, Darwin identified a further two mechanisms which are important factors in the evolution of sex differences (sexual dimorphism): intrasexual selection (involves competition with those of the same sex over access to mates) and intersexual choice (discriminative choice of mating partners).[33] Life history theory[34] (that includes Parental Investment Theory)[35] provides an explanation for the above mechanisms and strategies adopted by individuals, leading to age disparity in relationships. Life history theory posits that individuals have to divide energy and resources between activities (as energy and resources devoted to one task cannot be used for another task) and this is shaped by natural selection.[36]