An overarching evolutionary theory which can provide an explanation for the above mechanisms and strategies adopted by individuals which leads to age disparity in relationships is called Life History theory, 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.Parental Investment Theory refers to the value that is placed on a potential mate based on reproductive potential and reproductive investment.These differences may be sexual, financial or social in nature. Socially, a society with a difference in wealth distribution between older and younger people may affect the dynamics of the relationship.Although the "cougar" theme, in which older women date much younger men, is often portrayed in the media as a widespread and established facet of modern Western culture, at least one academic study has found the concept to be a "myth".
This theory predicts both intrasexual selection and intersexual choice due to differences in parental investment; typically there is competition among members of the lower investing sex (generally males) over the parental investment of the higher investing sex (generally females) who will be more selective in their mate choice.
A study released in 2003 by the United Kingdom's Office for National Statistics concluded that the proportion of women in England and Wales marrying younger men rose from 15% to 26% between 19.
Another study also showed a higher divorce rate as the age difference rose for when either the woman was older or the man was older.
The study found very few instances of older women pursuing much younger men and vice versa.
Within sexual selection Darwin identified a further two mechanisms which are important factors in the evolution of sex differences (sexual dimorphism): intrasexual selection (involve competition with those of the same sex over access to mates) and intersexual choice (discriminative choice of mating partners).