The self-expansion model proposes that individuals seek to expand their sense of self by acquiring resources, broadening their perspectives, and increase competency to ultimately optimize their ability to thrive in their environment.^[1][2][3] It was developed in 1986 by Arthur Aron and Elaine Aron to provide a framework for the underlying experience and behavior in close relationships. The model has two distinct but related core principles: the motivational principle and the inclusion-of-other-in-self principle. The motivational principle refers to an individual's inherent desire to improve their self-efficacy and adapt, survive, and reproduce in their environment. The inclusion-of-other-in-self principle posits that close relationships serve as the primary way to expand our sense of self as we incorporate the identities, perspectives, resources, and experiences of others as our own through these relationships.

One of the underlying themes of self-expansion is that individuals have a very basic motive to self-expand. Unlike Maslow's hierarchy of needs theory, it is omnipresent: only low-level needs are met before high-level needs will appear.^[4] The need of self expansion occurs even if there is a basic need that is unsatisfied.^[5]  Some researchers believe that self-expansion, like hunger and thirst, originates from the basic physiological needs in the nervous system,^[6] and is associated to the reward system in the brain.^[7] Some researchers also believe that the motivation for self-expansion is partly rooted in social approval and acceptance.^[8] Self-expansion may be conscious or unconscious. People may sometimes realize a sense of "self-expansion" or strive to achieve a goal that can promote self-expansion, but most of the time, self-expansion is an unconscious motivation^[5]

Self-expansion is the desire to enhance an individual's potential efficacy.^[9][10] Motivational models often refer to self-efficacy as one's belief that they are competent and can achieve specific goals.^[11] Within the self-expansion model, potential efficacy is used instead, as it only refers to obtaining resources that will make goal attainment possible. Achievement of this goal is a secondary concern.^[12]

However, the motivation to self-expand still does influence attraction to others for a potential close relationship. Aron and Aron suggest that our attraction is broken down into two components based on Rotter's value-expectancy approach.^[9]

• Desirability is the perceived total amount of self-expansion possible from a potential close relationship.
• The second factor, probability, refers to the likelihood that the close relationship with the individual can actually be formed. It can also be conceptualized as the likelihood that self-expansion will occur. Consequently, individuals will seek a partner that has high social status and a greater number of resources. However, to maximize self-expansion, consideration is also given to how likely this person will be loyal and desires to be in the close relationship.

The self-expansion model was originally established to explore the question, "Why do people develop and maintain relationships?" It attempts to describe people's feelings and actions in intimate relationships,^[9] therefore related research is mostly based on intimate relationships and the motivation behind the relationship. In 2002, Lewandowski and Aron designed the Self-expansion Questionnaire (SEQ) to explore expanding self and relationship.^[13] The self-expansion questionnaire (SEQ) is composed of several key components from the self-expansion model.^[13] It contains a total of 14 items to measure the degree of increase in the knowledge, skills, and abilities that an individual has experienced from his or her partners. There are three representative questions: “How much does your partner help to expand your sense of the kind of person you are?”; “How much has knowing your partner made you a better person?”; and “How much do you see your partner as a way to expand your own capabilities?”.^[14]

The second principle of the self-expansion model is that people use close relationships to self expand by including the other in the self. The self is often described as the content or the knowledge of who we are.^[15] Others could be interpreted as individuals. Some studies have also found that individuals will also include groups^[16][17] or communities.^[18]

According to Aron and Aron, when entering a close relationship a person should perceive that the self and other should begin to overlap by including aspects of the other in the self.^[9] More specifically, after people include others into themselves, they feel that they have the same resources, ideas, and identity as others, and they will think that the resources, ideas, and identity that others have gained or lost are also what they have gained or lost.^[5] These new resources lead to greater inclusion of the other in the self by also incorporating the other's perspectives and identities in the self.^[19]

Aron, Aron, Tudor and Nelson conducted several classic studies that scientifically demonstrated that we include the other in the self.^[20]

• In one experiment, participants were more likely to distribute money equally between the self and the close other in comparison to distributing the money between oneself and a stranger. The sharing of resources was suggestive of including the self in the other.
• In a second experiment, participants were more likely to remember more nouns for a stranger than a close other (one's mother). This supported the IOS phenomenon, as participants were more likely to take the perspective of the close other thus not being able to remember descriptive nouns of that person.
• In a final experiment participants were required to make yes/no decisions on whether certain traits belonged to themselves. Decisions on traits that were different between a participant and a close other had longer reaction times than decisions on traits that were different between a participant and a stranger.

It was suggested that the increased confusion between the self and the close other was directly related to integrating the other in the self. The degree of closeness in the relationship affects the self and other reaction studies.^[21] As two individuals become closer, there is greater confusion and therefore a longer reaction time. As a result, as closeness of a relationship increases, there will be a greater inclusion of the other in the self.

The Inclusion of the Other in the Self Scale (IOS Scale)^[22] is one of the most frequently used tools to assess this phenomenon. The scale consists of seven Venn diagram-like pairs of circles that vary on the level of overlap between the self and the other. Respondents are asked to select the pair of circles that best represents their current close relationship. Several studies have showed that this measurement tool is effective in getting accurate depictions of the amount of closeness and the inclusion of the other in the self.^[23][24]

IOS has also been assessed with the Continuous IOS, a Java-based applet suitable for online surveys that measures IOS on a continuous scale from 0-100^[25] Participants are instructed to use their mouse to move one of the circles (typically labeled "self") towards the other (typically labeled "other") until the degree of overlap best describes the relationship in question. The IOS Scale has also been adapted to measure inclusion in other contexts, for example community connectedness via the Inclusion of Community in the Self Scale.^[18] The adaptability of IOS is broad as demonstrated by recent versions substituting target with "X."^[24]

The idea of including the other in the self has been extended to include an entire ingroup in the self.^[26] An ingroup is an interdependent set of individuals with which a person identifies. The individual believes he or she is a member of this group.^[15] In fact, several academic groups have found similar findings in the me/not me reaction time paradigm at a group level.^[27] Participants showed a slower reaction time for traits that were incongruent between the self and ingroup. This was in comparison to quicker reaction times for traits that were congruent between the self and ingroup. The slow reaction times were consistent with the inclusion of the ingroup in the self claim as it suggested that the individual had included group characteristics in the self. As a result, there was difficulty recalling if a trait belong to the self or ingroup.

Several researchers have examined the role of ingroup identification (i.e. a person's prolonged psychological connection to an ingroup) and self-expansion. In fact, Trop and Wright refined the meaning of ingroup identification and believed it was analogous to the inclusion of the ingroup in the self.^[28] The authors found that the degree of connectedness to the ingroup will affect confusion of self-descriptors. People who highly identified with an ingroup showed slower reaction times for self-descriptors that did not relate to the ingroup (this is consistent with previous findings). However, low ingroup identification lead to no differences in reaction times between whether or not the self-descriptors were also descriptive of the ingroup. This demonstrated that the level of identification with an ingroup can be conceptualized as the degree to which we will include the ingroup in the self.

The central motivation for including the ingroup in the self parallels the self-expansion model at the interpersonal level. The self-expansion model suggests that we are strongly motivated to expand ourselves by including the other in the self. This occurs when an individual incorporates the other's perspectives, identities and resources. Likewise, it has been proposed that including an ingroup in the self, or ingroup identification, is partly influenced by the self-expansion motive. Inclusion of the ingroup's perspectives and resources can increase one's confidence in completing a variety of goals. Thus, a group's attractiveness is often based on the potential for self-expansion. A group with higher social status and a greater amount of potential resources is more likely to be included in the self.^[29]

Tropp and Wright created an instrument to measure the extent that an individual includes the ingroup in the self.^[28] The Inclusion of the Ingroup in the Self Scale (IIS Scale) was based on the Inclusion of the Other in the Self Scale. Similar to the original scale, seven pairs of circles vary on the degree of overlap between the self and the particular ingroup. The scale has been well validated and the degree of inclusion of the ingroup in the self is said to capture the sense of ingroup identification.

An abundance of research focuses on the negative interactions between different groups (e.g. negative attitudes, intolerance, discrimination). However, there is sparse evidence to explain the positive aspects of intergroup relations such as reduction of prejudice. The contact hypothesis (by Gordon Allport) is one area of psychology that focuses on positive aspects of intergroup relations.^[30][31] The hypothesis suggests that when there is cooperation, equal status, common goals and authority support then contact between members of different groups can result in reduced negative attitudes. In addition, positive emotions between intergroup members was said to be of utmost importance as it would lead to positive attitudes which, in turn, would generalize to the entire out group.^[32] An out group is set of individuals with which the individuals does not identify.^[15] It was unclear how this attitude generalization actually happened. Recently, the process of including the out group in the self was used as an explanatory mechanism for this generalization.^[26]

Including the out group in the self is based on the similar self-expansion notion of including the other or ingroup in the self. As a person becomes a friend with an out group member, the aspects of the out group is included in the self when that group is made salient. Essentially, representation of the out group and its identity is shared with our representation of the self. Including the out group in the self can vary; a person may actually become a member of an out group.^[26] However, in certain circumstances this is virtually impossible and we simply psychologically identify with the group even though we are aware that we are not part of it. Upon inclusion of the out group in the self, we now give that group several benefits. These benefits include taking pride in the group, sharing resources, and positive biases in causal explanations of the group. These benefits all increase the likelihood for reduced out group prejudice, hence its use as a mechanism for the contact hypothesis.

It is important to note that the focus of the inclusion of the out group in the self is initially at the interpersonal level (i.e. between individuals).^[33] The interaction needs to have interpersonal closeness for inclusion of the self in the other to occur. Consequently, a decategorized approach to contact should be used where the focus is on personal identities instead of group identities. By focusing on personalization, there is a greater chance for the development of closeness. Eventually, group membership needs to become available so that an individual can now include the out group in the self as well. However, it is believed that group membership will become more central in a natural manner as the close relationship develops.

Empirical evidence seems to support the inclusion of the out group in the self hypothesis. In one study white women were paired off with either another white woman (ingroup member) or a Latina woman (out group member).^[33] The pairs of women met over an extended period of time completing different activities together, which led to a measurable close friendship. Women with an intergroup friendship were more likely to have positive intergroup attitudes, less likely to endorse anti-minority policies and less likely to demonstrate intergroup anxiety. The study suggests that the intergroup close relationship led to improved attitudes towards the entire out group as suggested by the inclusion of the out group in the self mechanism. Another study also found that the level of inclusion of the out group in the self would affect the amount of decreased prejudicial attitudes.^[34] Further, simply including a friend in the self who has a close relationship with an out group member can decrease out group prejudiced attitudes. Knowing that a friend includes an out group member in the self allows for the individual to include that entire out group in the self. In turn, this also leads to positive attitudes about the out group.

It may be the case that individuals want to make friends with out group members (instead of oppress and mistreat the out group) because of the self-expansion motive.^[26] Based on Aron and Aron's original work, people want to expand the self and an optimal way of doing so is to make close friendships that give the opportunity for increased perspectives, identities and resources. People who are most similar to ourselves provide a diminished capacity for self-expansion. As a result, an individual may turn to out group members for friendship because they are different from one's self-concept. These differences allow for a greater likelihood to increase resources, identities and perspectives, which is consistent with the self-expansion motive. A recent study has shown that, consistent with this idea, priming high self-expansion motivation enhances out group self-expansion and the quality and outcomes of out group interactions (e.g. greater self-efficacy, reported closeness, and self-growth).^[35]

Self-expansion motives can explain why people may appreciate intergroup contact, however, it can also provide explanations for why we avoid this intergroup contact. People may be cautious of self-expansion due to a sense of self-loss. As we self-expand in one area we may put ourselves at risk of losing aspects of the self in another area. Consequently, people may be fearful of creating a close relationship with an out group member as this may trigger animosity from original ingroup members. Often individuals must attempt to balance the potential benefits of including the out group in the self with the potential loss of ingroup friends and the associated resources. If the self-loss outweighs the self-expansion, it is possible for a decrease in perceived self-efficacy.^[29]

The second barrier to self-expansion is the risk that an overabundance of self-expansion might occur in too short a period.^[36] The accumulation of new resources and perspectives in our self-concept leads to a need for self-integration (i.e. combining different resources, identities and perspectives into single overarching self-concept). An excessive amount of self-expansion without proper self-integration can be quite stressful (e.g. moving to a new city, or starting a new job). It has been suggested that when a person is socially stable, self-expansion via an out group member is most likely to be successful. Consequently, the likelihood for cross-group contact and the inclusion of the out group in the self is dependent on the degree of self-expansion in other domains. Expansion = motivation and believing.

More recently,^[when?] self-expansion research has begun to shift away from investigating self-expansion in a social context (e.g. romantic relationships)^[37] and instead has focused on self-expansion processes and outcomes of self-expansion at the individual level.^[38][39][40] Research has also focused on self-expansion in domains such as the workplace.^[41] Results of these research studies has shown that self-expansion can (and does) occur at the individual level (e.g. through hobbies and spiritual experiences) and in workplace settings. Similar to findings from social self-expansion literature, individual self-expansion also has positive effects and includes the same processes (e.g. motivation, self-efficacy). Self-expansion has also been presented theoretically within a framework of self-concept change.^[42] That is, self-concept change can be thought of as occurring along two independent dimensions: valence (positive vs. negative content) and direction of change (increase vs. decrease in content) and self-expansion represents one of the four possible processes of self-concept change (increasing positive content).

Self-expansion usually begins with attraction toward another. Falling in love provides an opportunity for rapid self-expansion as there is a desire to unite with the person you love.^[3] Studies have shown that perceived similarity and likeness can promote interpersonal attraction.^[3] People tend to prefer others that are similar to them, which goes against the motivation to increase expansion opportunities.^[43][3] While the perceived similarity in interests, background, and values is found to be an important factor affecting attraction, dissimilarity in partners is an avenue for expansion due to the provision of novel resources, opinions, and identification.^[44][45] A study found that providing information about a high likelihood of developing a relationship with the target person diminished and somewhat reversed the similarity-attraction effect for men in particular.^[46][44] In this case, men were more likely to select dissimilar partners as they providing more opportunity for expansion of the self. People may generally be reluctant to pursue dissimilar others despite the opportunity for self-expansion. They are likely to assume that the dissimilar other would not reciprocate their feelings and would expect no expansion to occur.^[47] For established couples, research shows they can experience different levels of motivation for self-expansion throughout their relationship, and these findings have been replicated in cross-cultural samples.^[44] As relationships continue to change and evolve, the degree to which they foster expansion and growth may vary in the future. A type of confirmation bias emerges such that those that expect future self-expansion through their relationships are more likely to engage in self-expanding activities.^[48] Potential for future expansion is a strong predictor of relationship satisfaction and commitment.^[44]

Research shows that when couples participate in some self-expansion activities, these activities can increase relationship satisfaction and improve relationship quality. However, expansion can also reduce negative outcomes such as relationship boredom. In one study, dating college students and married couples attributed boredom in their relationships to low novelty and stimulation.^[44] This was also associated with low pleasure and arousal in the relationship. When participants were primed with boredom in another study, they were more inclined to seek out new activities.^[44] This follows the first principle of the model which posits that individuals possess an inherent motivation to expand themselves. Sexual desire can be affected by relationship boredom. Those that report high sexual desire for their partners are more likely to have sexual and overall satisfaction in their relationships.^[44] Consensually non-monogamous individuals that have more than one sexual or romantic partner at the same time have more opportunities for self-expansion, suggesting positive implications for this relationship arrangement.^[44] Transitioning to an open relationship can be a novel and exciting experience for a couple that can increase levels of passion as posited by the research. There are several non-sexual opportunities as well when multiple partners are involved, such as exploring different interests and learning new perspectives.^[44]

Romantic relationships that do not provide sufficient opportunity for individual self-expansion may increase the inclination to perceive and focus on alternative partners.^[44] If this occurs to a great extent, it could lead to negative relational outcomes for an exclusive, monogamous relationship such as low relationship satisfaction/stability and a higher susceptibility to infidelity.^[44] In one study, participants that reported lower self-expansion in their relationships were more likely to enjoy interacting with a computer program simulation that was shown to be more self-expanding.^[44] In such circumstances, individuals are less likely to employ motivational and perceptual biases that protect against the influence of attractive alternatives. A mediational analysis demonstrated that attention to alternatives explains the observed relationship between self-expansion and infidelity.^[44] In another study, fMRI data revealed less brain activation in response to a series of attractive faces when participants were primed to remember moments of self-expansion in their current relationships, suggesting that self-expansion can discourage them from attending to attractive alternatives.^[44] Infidelity is a severe relational transgression that could lead to the dissolution of the relationship.^[49][50] Ending a relationship is dependent on many key factors such as "improvement of the self" and "hindering self-improvement".^[44] The self-expansion opportunities provided in a relationship can also affect the individual after breaking up. Loss of a relationship that does not promote expansion is found to have positive effects on the individual as it could provide new, previously restricted opportunities for growth.^[44] However, the individual may experience "self-contraction" (loss of self) if the relationship did provide sufficient expansion.^[51][44]

Relationships serve as the primary means of self-expansion. However, personal or non-relational self-expansion can still occur through novel and challenging individual experiences (e.g. learning a new language) resulting in several intrapersonal benefits.^[3][52] Notable behavioral changes caused by individual self-expansion include smoking cessation, weight loss, and better physical health.^[44] Using an experience sampling design, researchers showed that increases in personal self-expansion in the same individual were correlated with higher passion in the relationship, while low levels of passion were observed for high levels of self-expansion across participants.^[53] This suggests that growth experienced through non-shared activities can fulfill individual needs but could reduce passion for one's partner. Another contributing factor is the level of support received by the partner. A study of retired married couples showed that prior support from partners for individual growth predicted happiness and satisfaction during retirement years.^[44] Relationship satisfaction for ongoing couples is increased when partners are encouraging non-relational self-expanding activities. This occurs as individuals believe their partners are facilitating their expansion and associate them with that expansion.^[3]