In the 1996 romantic comedy Jerry Maguire, when Jerry (the protagonist) tells Dorothy (his love interest) "You complete me," many viewers instantly get emotional. But beyond tugging at our heartstrings because of his romantic gesture, Jerry communicates something more fundamental to Dorothy. His message conveys the notion that romantic relationships can change how we see and understand ourselves. Jerry's proclamation of love suggests that his sense of identity has been molded, shaped, and "completed" through his relationship with Dorothy.

In relationship science terms, Jerry describes self-expansion—how romantic relationships change people's sense of self for the better. For example, partners might introduce one another to new musical acts, books, or hobbies that they incorporate into their identities. Self-expansion brings the couple closer to one another, and provides each of them with a larger and more diverse sense of self.

People in self-expanding relationships report that they have better relationships, love their partners more, are more satisfied sexually, do more to maintain their relationships, and are less vulnerable to cheating. They also reap several personal benefits such as higher self-esteem and greater self-efficacy (the belief that one can overcome challenges). 

Building on these findings, we theorized that self-expansion broadly helps people develop and strengthen psychological resources to cope with stress. If so, people in self-expanding relationships may have better mental health and experience fewer depression symptoms. We conducted four studies to test this idea. In the first two studies, we recruited a total of 610 romantically involved adults and asked them to complete measures of self-expansion and depression symptoms. We measured self-expansion with items such as "How much does your partner help you to expand your sense of the kind of person you are?" We measured depression by asking people how often they experienced a range of symptoms, such as "feeling down, depressed, or hopeless."

Overall, we found that people in self-expanding relationships felt less depressed. These results held even after considering many other variables that are related to depression. Of course, this is but one snapshot in time, and this association might be due to some unmeasured personality trait that predicts both self-expansion and depressive symptoms.  We reasoned that if self-expansion reduces depressive symptoms, then on days that people find their relationship more self-expanding than usual, they would feel less depressed than they usually do.

In Study 3, we explored this process on a day-to-day basis. We had 100 couples complete measures of self-expansion and depression for 14 consecutive days. As we expected, participants felt less depressed on days their relationships were more self-expanding than usual. However, the direction of this association wasn't clear.  Depressive symptoms might lead to reduced self-expansion, or self-expansion might lead to less depression.

Study 4 examined the link between self-expansion and depressive symptoms over 9 months, to better home in on how self-expansion and depressive symptoms are related. One hundred nine people completed measures of self-expansion and depression at the beginning of the study and again nine months later. People whose relationships became more self-expanding over nine months reported fewer depression symptoms at the end of the study, consistent with the idea that self-expansion has cumulative benefits for mental health over time.

In sum, relationships that expand people's sense of themselves also help them flourish, by feeling less depressed. Finding that person who "completes" you (or, more accurately, "expands" you) may be an important step to improving your mental wellness.

For Further Reading

McIntyre, K. P., Mattingly, B. A., Stanton, S. C., Xu, X., Loving, T. J., & Lewandowski, G. W. (2023). Romantic relationships and mental health: Investigating the role of self-expansion on depression symptoms. Journal of Social and Personal Relationships, 40(1), 3-28.

Aron, A., Lewandowski, G., Branand, B., Mashek, D., & Aron, E. (2022). Self-expansion motivation and inclusion of others in self: An updated review. Journal of Social and Personal Relationships, 39(12), 3821-3852.

Mattingly, B. A., Tomlinson, J. M., & McIntyre, K. P. (2020). Advances in self-expansion. In L. V. Machia, C. R. Agnew, & X. B. Arriaga (Eds.), Interdependence, Interaction, and Close Relationships (pp. 225–245). Cambridge University Press.

Kevin P. McIntyre is a Professor of Psychology at Trinity University. His research examines how romantic relationships affect and are affected by individuals' self-concepts. 

Brent Mattingly is an Associate Professor of Psychology at Ursinus College. He studies the interplay between close relationships and individuals' sense of self.