Males and femals value different characteristics in a partner. Buss was an Evolutionary Psychologist who based his idea's on Charles Darwin's theory of evolution.
The theory suggested species develop and change accordingly to their environment. The species best equipped to survive would mate and pass on genetic characteristics.
Over time species develop a preference for selecting a mate for reproduction who had the best survival characteristics.Based on evolution buss suggested Males and Femals look for differant characteristics when choosing a partner.
Trivers suggested males and females have differant selection for a mate, because they have differant levels of parental investment, women have more investment in reproduction so look for men with strength and reliablility. Men look for women who are attractive which indicate good genes and indicate fertility. Thornhill suggested males select femals who are younger as its indication to fertility. Females choose males who are old demonstrating power, caring and prospects.