The literature on small-group formation contains two major hypotheses about the determinants of attraction between individuals. The first of these—the similarity thesis—suggests that people are drawn together because of similarities in their personal characteristics (attitudes, ages, interests and so forth). Theodore M. Newcomb's study of friendship formation at college (The Acquaintance Process, 1961) supports this view. The second hypothesis maintains that interpersonal attraction takes place on the grounds of complementarity of characteristics between individuals. For example, Robert Francis Winch's investigation of married couples (Mate-Selection: A Study of Complementary Needs, 1958) suggested that ‘social needs’ (such as deference, aggressiveness, and exhibitionism) should be complementary rather than similar, if marriages were to work. If one partner was low in a particular attribute then the other should be high. Furthermore, certain combinations of attributes were favoured, such as high deference and high dominance. Later modifications of this thesis took into account additional variables (such as mutual gratification of social needs and the social context of relationships).