Self-expansion motivation and inclusion of others in self: An updated review

In this paper we review the self-expansion model in the context of close relationships, focusing primarily on work in the last 20 years, considering throughout variation in our samples across cultures and other demographics—both in existing studies and in potential implications for future research. The self-expansion model has two key principles. The first half of the paper focuses on the motivational principle: The model theorizes that people have a fundamental desire to expand the self—that is, to increase their self-efficacy, perspectives, competence, and resources, and this often occurs through relationships in general. The second half of the paper focuses on the inclusion-of-other-in-the-self principle, in that a major means of self-expansion is through close relationships, when one’s partner’s identities, perspectives, skills, and resources become to some extent “included in the self” as also one’s own. For each principle we briefly describe its foundational research support and then explore the extensive, significant work of the last 20 years substantially expanding and deepening the implications of the model. The majority (although with some interesting exceptions) of studies have fallen short of testing the universal breadth of the model. As we review the research, we consider where the studies were conducted and with what kinds of populations. Where there are data from diverse populations, the overall pattern of results are generally similar. However, there were individual differences found within the populations studied, such as in attachment style, that affected the operation of both principles. Since there are well known differences in the distribution of such individual differences across populations of many types, it is quite likely that while the basic patterns may not differ, future research will show different degrees of operation in different populations.