Another important factor that influences attraction is similarity… elementary school students prefer other children who perform about as well as they do in academics, sports, and music, and best friends in high school tend to resemble each other in age, race, year in school, and grades. Attributes such as race, ethnic origin, social and educational level, family background, income, and religion do affect attraction in general and marital choice in particular. Also relevant are behavioral patterns such as the degree of gregariousness and drinking and smoking habits. Like pairs with like, and heiresses rarely marry the butler except in the movies.

-P.536-537, Psychology, 8th ed., by Gleitman, Gorss, and Reisberg.