A study conducted by David Buss investigated sex differences in mate preferences in 37 cultures with 10,047 participants.In all 37 cultures it was found that males preferred females younger than themselves and females preferred males older than themselves.Age-disparity relationships have been documented for most of recorded history and have been regarded with a wide range of attitudes dependant on sociocultural norms and legal systems.
This theory is directly relevant and compatible with the two already mentioned theories Life History and Parental Investment.
Although there is a number of reproductive advantages to short term mating, males still pursue long term mates and this is due to the possibility of monopolising a female's life time reproductive resources.
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.
However, human males tend to have more parental investment compared to mammal males (although females still tend have more parental investment).