To a greater extent than in the past, union formation is currently considered an issue that is mainly the concern of the partners in the relationship (Shorter, 1977). Although the opinions of parents and friends still do matter, in the end the partners themselves decide whether they want to live together and, if so, whether they want to get married or not. This shift is said to be the result of the progressing individualization in industrialized societies (Lesthaeghe & Surkyn, 1988). Due to this, individuals are less dependent than in the past on customs and norms prevailing in their social environment, and can decide fairly autonomously on personal life choices. If partners are fairly autonomous in making their union formation decisions, one would expect a high degree of correspondence between the intentions of young adults regarding union formation and their actual behavior. However, hardly any research has been conducted on this issue. Therefore, the first question to be dealt with here concerns the extent to which the intentions of young adults regarding the timing of union formation and regarding the type of union chosen are actually realized. There is a large literature on social factors influencing the timing of marriage (e.g., Blossfeld & Huinink, 1991; Oppenheimer, 1988). However, empirical research on factors influencing the choice between unmarried cohabitation and marriage is much more limited (Hoem, 1986; Liefbroer, 1991a; Rao, 1990; Thornton, 1991). Social-psychological models linking attitudes and behavior, such as Fishbein and Ajzen's (1975) theory of reasoned action, stress that the influence of such social factors is always mediated by the behavioral intentions of people. Applying this to the issue of union formation choice, one might expect that union formation intentions mediate the impact of social factors on union formation behavior. Our second aim is to test this hypothesis. In doing so, we will not include the full range of possible social factors, but concentrate on the impact of a number of life course related factors. Two main concepts from life course sociology will be used to structure the discussion, namely developmental readiness and parallel careers. HYPOTHESES RELATIONSHIP BETWEEN UNION FORMATION INTENTIONS AND BEHAVIOR The relationship between attitudes and intentions of individuals on the one hand, and their behavior on the other hand, has been extensively studied, mainly within social psychology. Fishbein and Ajzen (1975) emphasize that, in order to predict future behavior, it is better to inquire about concrete intentions than about general attitudes and values underlying behavior. Behavioral intentions appear to be good predictors of behavior, particularly if it concerns relatively 'simple' behavior that is to be realized soon, and of which the realization is fully under the individual's control. If a long period of time elapses between determining the behavioral intention and the actual behavior, or if the behavior is partly outside the individual's control, it appears necessary to pay more attention to the social context (Ajzen & Madden, 1986; de Jong Gierveld & Liefbroer, 1988; Liska, 1984). Numerous demographic applications of Fishbein and Ajzen's (1975) model of reasoned behavior can be found. However, most of these applications are related to having children (e.g., Bracher & Santow, 1991). Applications to the union formation process are scarce. The Netherlands Fertility Survey, conducted by the Netherlands Central Bureau of Statistics (Beets, 1987; Van de Giessen, 1986), forms an exception to this rule. In 1982, females who were not living with a partner were asked about their future union formation plans. During reinterviews in 1985, the majority of the women were found to have acted in accordance with their stated expectations. Sixty-nine percent of those who had planned to get married within 3 years had actually done so, while this percentage was only 18 among those women who had not intended to get married. …