Another person’s life can quietly become part of our own. A friend introduces us to music we would never have chosen. A partner brings us into a new community, teaches us a skill, or offers a way of seeing a problem that continues to guide us when they are absent. With time, these influences may no longer feel borrowed. They become part of what we know, what we can do, and how we understand ourselves.

Self Expansion Theory offers a psychological account of this change. The model proposes that people are motivated to increase their capacity to pursue goals. Close relationships can contribute to that expansion because another person’s resources, perspectives, and identities may become partly included in the self (Aron et al., 2022; Aron, Mashek, & Aron, 2004).

This does not mean that every relationship enlarges life or that greater overlap is always healthier. A relationship can also restrict choice, weaken valued identities, or make separation especially disorienting. The theory is most useful when it helps us recognize how connection can support growth without turning closeness into self-erasure.

How Self Expansion Theory Works in Close Relationships

Arthur and Elaine Aron first presented the self-expansion model in the 1980s to explain motivation and cognition within close relationships. Later formulations organize it around two related ideas. First, people seek to expand their potential efficacy, or their perceived capacity to accomplish goals. Second, close relationships offer one route to that expansion because people can gain access to another person’s resources, perspectives, and identities (Aron et al., 2022).

Self-efficacy usually refers to a person’s judgment that they can carry out a particular course of action (Bandura, 1977). Potential efficacy is broader. It concerns acquiring the knowledge, abilities, connections, and other resources that may make future action possible. We often seek growth before we know exactly how we will use it.

The model describes several kinds of expansion. Resources can include knowledge, skills, social connections, practical support, and access to opportunities. Perspectives are ways of interpreting events and experiencing the world. Identities include roles, group memberships, and other parts of self-definition. Through a close relationship, what first belongs to another person may become psychologically available to us as well (Aron et al., 2013; Aron, Mashek, & Aron, 2004).

Self-expansion is more specific than generic self-improvement. It is also different from stimulation for its own sake. A person may expand through learning, entering a community, developing a role, or encountering a demanding idea. Excitement often accompanies rapid expansion, but excitement is not the defining feature.

Including Another in the Self

The phrase including other in the self can sound as if two people lose their distinction. The theory proposes something subtler. A close other becomes represented within the mental organization of the self. Their outcomes may matter as though they were partly our own, and their knowledge or perspective may shape how we respond even when they are not present (Aron, Mashek, & Aron, 2004).

Imagine that one partner knows how to navigate an unfamiliar city while the other speaks the local language. Together, each person has access to abilities that neither possesses alone. As the relationship develops, the expansion may go beyond practical cooperation. One partner begins to notice architecture through the other’s interest. The other becomes more willing to enter conversations that once felt inaccessible. The relationship has changed what each person can perceive and attempt.

Aron, Mashek, and Aron (2004) describe a proposed process in which another person’s resources first become available through the relationship. Repeated interaction then reorganizes how self and other are represented. Some of the other’s perspectives and identities are incorporated, and that inclusion can make further sharing more likely. The authors present this as a theoretical process rather than a rigid sequence that every relationship must follow.

Inclusion Is Not Familiarity

We can know a person well without including them in the self. Familiarity comes from exposure and attention. Inclusion concerns whether another person’s resources and outcomes are experienced as psychologically connected to our own. We may be very familiar with a supervisor, neighbor, public figure, or adversary while feeling little closeness to them (Aron, Mashek, & Aron, 2004).

Inclusion Is Not Sameness

Closeness also does not require two people to become alike. Difference may offer opportunities for expansion precisely because another person knows, values, or experiences something we do not. A relationship organized around absolute similarity would offer fewer new perspectives and resources. The important question is not whether partners become duplicates. It is whether contact with each other changes the range of what each can understand and do.

Measuring Closeness With the IOS Scale

The best-known measure associated with the theory is the Inclusion of Other in the Self Scale. The IOS Scale presents seven pairs of circles labeled self and other, ranging from separate to highly overlapping. A participant selects the pair that best represents a particular relationship. In the original validation studies, this single-item pictorial measure showed evidence of reliability and several forms of validity across romantic, friendship, and experimentally created relationships (Aron, Aron, & Smollan, 1992).

The image is intuitive, but the score has a narrow purpose. The IOS Scale measures a person’s perceived closeness or self-other inclusion. It does not directly measure the process of self-expansion, nor does it assess compatibility, safety, secure attachment, or relationship stability. Two people can report substantial overlap while struggling with relationship boundaries or a fear of engulfment. Another pair may preserve considerable individuality while sharing a dependable and caring bond.

Researchers approach the theory from several angles. Some ask people how much a relationship has opened new experiences or abilities. Others use timed tasks to see whether information about a partner is processed more like information about the self. Each method captures a different part of the model. A closeness score, an activity rating, and a cognitive task may be related, but they do not measure exactly the same psychological process (Aron et al., 2013; Emery, Hughes, & Muise, 2025).

How Self Expansion Changes Across a Relationship

New Relationships and Rapid Expansion

The beginning of a relationship offers unusually concentrated opportunities for expansion. A new partner brings unfamiliar stories, habits, abilities, relationships, and possible futures. Learning about the person and building a shared world can rapidly add content to the self-concept. The novelty and pace of this process may help explain why falling in love often feels energizing (Aron et al., 2013).

Prospective studies summarized by Aron and colleagues found that people who fell in love showed greater change in spontaneous self-descriptions than people who did not. Their descriptions contained more words and more distinct self-categories, alongside changes in self-efficacy and self-esteem. These findings support the idea that a developing relationship can alter how a person experiences the self (Aron et al., 2013).

The intensity of early expansion does not establish compatibility, safety, or long-term stability. Rapid change can make a relationship feel important before partners understand how they handle responsibility, conflict, power, or commitment. Self-expansion measures were not designed to answer those questions.

Familiarity and Slower Self Expansion

As partners come to know each other, the pace of expansion often changes. Familiarity is not a defect. It can bring trust, coordination, and the comfort of being known. Yet the relationship may no longer supply new information and possibility at the rate it did during courtship. A couple can remain loving while feeling less enlivened.

This change helps explain the theory’s interest in boredom. In two survey studies, couples who reported more shared exciting activities also reported greater relationship quality. Lower relationship boredom statistically helped account for that association. In three laboratory experiments, couples assigned to complete a brief novel and arousing task together reported larger immediate increases in experienced relationship quality than couples completing a mundane task. One experiment also included a no-activity comparison (Aron et al., 2000).

The studies do not show that boredom is the single cause of relationship decline. Their laboratory tasks lasted only several minutes, the samples came from the United States, and the researchers did not isolate the precise mechanism. Cooperation, positive emotion, arousal, and self-expansion could all have contributed. The results support a phenomenon under studied conditions, not a promise of lasting repair (Aron et al., 2000).

Shared Activities and Renewed Self Expansion

Self-expanding activities are usually described as novel, interesting, and challenging. A shared activity may give partners new knowledge, require unfamiliar coordination, or reveal a side of one another that routine rarely brings forward. Because the expansion occurs in the partner’s presence, some of the vitality may become associated with the relationship (Aron et al., 2013).

The word arousing in early studies sometimes led to a narrow interpretation: couples need thrilling or physically intense experiences. Tomlinson and colleagues tested self-expansion and physical arousal separately across four studies involving married couples, friends, and individuals reflecting on close relationships. Self-expansion predicted relationship and individual outcomes when arousal was controlled, while arousal did not show the same consistent pattern. Physical arousal therefore did not appear necessary for the measured benefits in those studies (Tomlinson et al., 2019).

The activity need not be dramatic. Partners might learn an unfamiliar recipe, explore a neighborhood, attend a lecture, begin a small creative project, or try a skill neither has mastered. What matters is whether the experience feels meaningfully new or challenging to these people. The same plan may be expansive for one couple, routine for another, and overwhelming for a third.

Research with established couples adds another dimension. In diary and longitudinal studies, Muise and colleagues found that greater daily self-expansion with a partner was associated with greater sexual desire and with indicators of relationship and sexual well-being. Perceived partner responsiveness helped explain part of this association. A weekend activity study increased reported self-expansion, but its effects across the other outcomes were less uniform than a simple prescription would suggest (Muise et al., 2019).

These findings make adding novelty a reasonable invitation, not a relationship requirement. Quiet companionship is not an inferior form of love. Partners also differ in health, money, time, caregiving demands, tolerance for uncertainty, and access to opportunity. Newness imposed through pressure is unlikely to feel like shared growth.

When Relationship Change Helps or Harms the Self

The fact that a relationship changes someone does not tell us whether the change is beneficial. Mattingly, Lewandowski, and McIntyre (2014) organized relationship-induced self-change along two questions: Is something being added or lost, and is that content experienced as positive or negative? This produces four possible patterns.

Two patterns describe changes people generally experience as improvements. Self-expansion means gaining a valued quality, such as confidence or a useful skill. Self-pruning means shedding an unwanted quality, such as a habit a person genuinely wants to leave behind.

Two other patterns describe losses or unwanted additions. Self-contraction means losing a valued part of oneself, while self-adulteration means taking on a quality one experiences as harmful or unwanted. Across three studies, the positive forms of change were associated with better relationship and individual functioning, while the negative forms were associated with poorer functioning. Because these studies were largely correlational, they reveal a pattern but do not establish what caused what (Mattingly, Lewandowski, & McIntyre, 2014).

The four patterns help separate change from growth. A partner who encourages a person to become more confident may support expansion. A relationship that helps someone stop a destructive habit may support pruning. By contrast, giving up valued friendships or ambitions may reflect contraction, while becoming more dishonest or cruel within a relationship may reflect adulteration. Not every influence fits neatly into a category, and partners may judge the same change differently. The categories are prompts for careful observation, not diagnoses.

Expansion also needs integration. The early theory proposed a rhythm in which people take in new experiences and then organize them into a coherent sense of self. A recent review found that research has concentrated far more on expansion than on this integration phase. Newness that cannot be absorbed may remain merely novel, and rapid change without time for integration may feel confusing rather than enlarging. These possibilities remain important theoretical questions rather than settled conclusions (Emery, Hughes, & Muise, 2025).

Research on relationship dissolution shows why separation can feel like a contraction of the self. Shared friendships, routines, interests, and future plans may no longer feel available. This can explain some of the disorientation of a breakup without implying that maintaining the relationship would have been healthier (Aron et al., 2022).

Healthy expansion widens a person’s life. It does not require shrinking friendships, silencing convictions, surrendering privacy, or allowing another person to decide which parts of the self may survive. Research on intimacy emphasizes perceived partner responsiveness: feeling understood, accepted, validated, and cared for helps disclosure become closeness rather than exposure (Laurenceau, Barrett, & Rovine, 2005).

Novelty is not repair. The activity studies did not test whether a weekend trip corrects coercion, chronic contempt, repeated betrayal, or violence. Even in relationships without abuse, shared activities cannot settle incompatible expectations about commitment, children, fidelity, substance use, money, or other central concerns. Maintaining a healthy relationship requires respect, responsiveness, accountability, and dependable behavior as well as moments of discovery.

Beyond Romantic Relationships

Although romantic relationships dominate the literature, self-expansion is not confined to couples. Friendship, family relationships, group membership, learning, travel, creative activity, and other pursuits may enlarge the range of resources and identities available to a person. The theory’s inclusion principle has also been applied to group identification and relationships across social boundaries (Aron et al., 2022).

A hobby can expand a person’s capability without involving a close partner. A relationship can expand the self through another person’s perspectives even when the pair is doing nothing conventionally exciting. Individual and relational expansion may support one another, but they are not the same process. Keeping several paths to growth also prevents one relationship from becoming the only doorway to possibility.

Limits of Self Expansion Research

Self-expansion theory has accumulated substantial research support, but the evidence is not equally strong for every claim or population. A 2022 review concluded that most studies had not tested the model’s proposed universal breadth. Where more diverse populations have been studied, broad patterns were often similar, yet attachment style and other individual differences altered how the processes operated. Cultural, demographic, and relationship variation therefore deserve more direct study rather than being treated as an afterthought (Aron et al., 2022).

Researchers do not yet have one agreed-upon way to measure self-expansion. Studies have asked about new experiences, rated changes in the self, or inferred expansion from activities and experimental tasks. A 2025 review noted that the most commonly used Self-Expansion Questionnaire began as an unpublished conference measure and has not undergone formal testing of how consistently and accurately it measures the construct. This makes it important to distinguish self-expansion from things that may prompt it, such as novelty, and from possible results, such as excitement (Emery, Hughes, & Muise, 2025).

Time remains a major gap. Diary studies capture meaningful day-to-day variation, and brief experiments help test immediate effects. They cannot fully show how expansion, integration, identity change, and relationship quality influence one another across years. Longer-term research is needed to determine when expansion slows, whether it can occur too quickly, how integration unfolds, and which changes endure (Emery, Hughes, & Muise, 2025).

The theory remains useful when treated as a supported framework with active questions rather than a universal formula for evaluating relationships.

Using Self Expansion Theory With Care

Self Expansion Theory is most useful as a lens for reflection. It invites us to notice how relationships have changed our capabilities and identities, where growth has slowed, and whether the life created together still leaves room for each person to develop.

A person might ask what they have learned through a relationship that now feels genuinely their own. Which communities or interests have opened? Which capacities have strengthened? They might also ask what has become harder to access. Have important friendships, values, ambitions, or forms of expression been crowded out?

Couples considering a new activity can begin with mutual choice and autonomy in relationships. Each person should be free to shape the plan, name limits, or decline. The activity should contain enough novelty or challenge to invite attention without creating avoidable threat. Success is not measured by how adventurous it looks from the outside. A small project can be expansive when it asks both people to learn.

The same reflection applies outside romance. We can seek relationships and pursuits that add to our understanding without requiring another person to carry the entire burden of our growth. Individual interests allow each person to bring new resources and perspectives back into the shared world.

A Few Words from Psychology Fanatic

Closeness leaves evidence in us. We borrow phrases, learn skills, enter unfamiliar rooms, and begin to see through another person’s eyes. Some of what enters through relationship becomes part of the self long after the moment has passed.

Self Expansion Theory gives language to this ordinary transformation. It also reminds us why love can feel both exhilarating and vulnerable. To include another person in the self is to gain access to a larger world, while accepting that the shape of our own life will change.

The wisest use of the theory does not demand constant excitement. It asks whether connection continues to make room for learning, agency, integration, and discovery. A healthy relationship can enlarge the life two people share while protecting the distinct life each person must still be able to call their own.

Associated Concepts

• Self Efficacy: Judgments about capability help clarify the theory’s broader idea of potential efficacy.
• Self Concept: The organized understanding of who we are can change as relationships add perspectives, roles, and identities.
• Emotional Intimacy: Responsive closeness allows people to know one another without making difference unsafe.
• Adding Novelty: New experience can stimulate attention and learning, although novelty alone does not define self-expansion.
• Maintaining a Healthy Relationship: Growth contributes to relationship quality alongside respect, accountability, repair, and dependable care.
• Relationship Dependence: Close relationships involve mutual influence, but healthy interdependence preserves meaningful autonomy.

References

Aron, Arthur; Aron, Elaine N.; Smollan, Danny (1992). Inclusion of Other in the Self Scale and the Structure of Interpersonal Closeness. Journal of Personality and Social Psychology, 63(4), 596-612. DOI: 10.1037/0022-3514.63.4.596.
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Aron, Arthur; Lewandowski, Gary W., Jr.; Branand, Brittany; Mashek, Debra; Aron, Elaine N. (2022). Self-Expansion Motivation and Inclusion of Others in Self: An Updated Review. Journal of Social and Personal Relationships, 39(12), 3821-3852. DOI: 10.1177/02654075221110630.
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Aron, Arthur; Lewandowski, Gary W., Jr.; Mashek, Debra; Aron, Elaine N. (2013). The Self-Expansion Model of Motivation and Cognition in Close Relationships. In Jeffry A. Simpson and Lorne Campbell (Eds.), The Oxford Handbook of Close Relationships (pp. 90-115). Oxford University Press. DOI: 10.1093/oxfordhb/9780195398694.013.0005.
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Aron, Arthur; Mashek, Debra J.; Aron, Elaine N. (2004). Closeness as Including Other in the Self. In Debra J. Mashek and Arthur Aron (Eds.), Handbook of Closeness and Intimacy (pp. 27-41). Lawrence Erlbaum Associates. ISBN: 9780805842852.
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Aron, Arthur; Norman, Christina C.; Aron, Elaine N.; McKenna, Colin; Heyman, Richard E. (2000). Couples’ Shared Participation in Novel and Arousing Activities and Experienced Relationship Quality. Journal of Personality and Social Psychology, 78(2), 273-284. DOI: 10.1037/0022-3514.78.2.273.
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Bandura, Albert (1977). Self-Efficacy: Toward a Unifying Theory of Behavioral Change. Psychological Review, 84(2), 191-215. DOI: 10.1037/0033-295X.84.2.191.
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Emery, Lydia F.; Hughes, Erin K.; Muise, Amy (2025). Self-Expansion Theory: Origins, Current Evidence, and Future Horizons. Social and Personality Psychology Compass, 19(9), e70082. DOI: 10.1111/spc3.70082.
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Laurenceau, Jean-Philippe; Barrett, Lisa Feldman; Rovine, Michael J. (2005). The Interpersonal Process Model of Intimacy in Marriage: A Daily-Diary and Multilevel Modeling Approach. Journal of Family Psychology, 19(2), 314-323. DOI: 10.1037/0893-3200.19.2.314.
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Mattingly, Brent A.; Lewandowski, Gary W., Jr.; McIntyre, Kevin P. (2014). “You Make Me a Better/Worse Person”: A Two-Dimensional Model of Relationship Self-Change. Personal Relationships, 21(1), 176-190. DOI: 10.1111/pere.12025.
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Muise, Amy; Harasymchuk, Cheryl; Day, Lisa C.; Bacev-Giles, Chantal; Gere, Judith; Impett, Emily A. (2019). Broadening Your Horizons: Self-Expanding Activities Promote Desire and Satisfaction in Established Romantic Relationships. Journal of Personality and Social Psychology, 116(2), 237-258. DOI: 10.1037/pspi0000148.
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Tomlinson, Jennifer M.; Hughes, Erin K.; Lewandowski, Gary W., Jr.; Aron, Arthur; Geyer, Rachel (2019). Do Shared Self-Expanding Activities Have to Be Physically Arousing? Journal of Social and Personal Relationships, 36(9), 2781-2801. DOI: 10.1177/0265407518801095.
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