Complementarity is a factor which affects attraction in romantic relationships. It is also referred to as ‘complementarity of needs’ and is the final stage of filter theory. It refers to how well two people fit together (complement one another) and meet each other’s needs. For example, young women may feel attracted to older men who have more economic resources and can provide for them. In return, the older men are able to go out with a younger woman, which demonstrates their virility in attracting such a good ‘catch’.