The evolutionary theory of human interpersonal attraction states that opposite-sex attraction most often occurs when someone has physical features indicating that he or she is very fertile. Considering that the primary purpose of conjugal/romantic relationships is reproduction, it would follow that people invest in partners who appear very fertile, increasing the chance of their genes being passed down to the next generation. This theory has been criticized because it does not explain relationships between same-sex couples or couples who do not want children, although this may have something to do with the fact that whether one wants children or not one is still subject to the evolutionary forces which produce them.

Another evolutionary explanation suggests that fertility in a mate is of greater importance to men than to women. According to this theory, a woman places significant emphasis on a man's ability to provide resources and protection. The theory suggests that these resources and protection are important in ensuring the successful raising of the woman's offspring. The ability to provide resources and protection might also be sought because the underlying traits are likely to be passed on to male offspring. Critics of this theory point out that most genes are autosomal and non-sex-linked (Gould, et al.)

Evolutionary theory also suggests that people whose physical features suggest they are healthy are seen as more attractive. The theory suggests that a healthy mate is more likely to possess genetic traits related to health that would be passed on to offspring. People's tendency to consider people with facial symmetry more attractive than those with less symmetrical faces is one example. However, a test was conducted that found that perfectly symmetrical faces were less attractive than normal faces.

It has also been suggested that people are attracted to faces similar to their own. Case studies have revealed that when a photograph of a woman was superimposed to include the features of a man's face, the man whose face was superimposed almost always rated that picture the most attractive. This theory is based upon the notion that we want to replicate our own features in the next generation, as we have survived thus far with such features and have instinctive survival wishes for our children. Another (non-evolutionary) explanation given for the results of that study was that the man whose face was superimposed may have consciously or subconsciously associated the photographically altered female face with the face of his mother or other family member. 

Increased female attraction to men in relationships
उत्क्रांतीवाद सिद्धांत
पूरकता Complementarity
कारणे Causes