Self-expansion model

The self-expansion model of interpersonal relationships proposes that people have a basic motivation to expand their physical influence, cognitive complexity, social or bodily identity, and self-awareness, and that the psychological reward from falling in love or an intimate relationship is whatever creates this "expansion of the self".
Relationships are an important area for self-expansion, via a process called inclusion of the other in the self, where aspects of a partner are incorporated into one's own self concept. Self-expansion can also take the form of having new and exciting experiences with a partner.
Besides romantic love, opportunities for self-expansion include learning, career, family, friendship, athletics, travel, artistic expression, politics, gossip, religion, and the experience of nature.
The model was developed in the 1980s by the psychologists Arthur Aron and Elaine Aron, as a variant of the reward theory of attraction developed by interpersonal attraction researchers of the 1960s and 1970s. A robust research literature on self-expansion has grown, supporting the idea that people do in fact self-expand by absorbing others into their self-concepts, and that self-expansion contributes to relationship well-being. However, some other features of the Arons' original theory remain untested.

Principles

Motivation

One of the underlying themes of self-expansion is that individuals have a very basic motive to self-expand. Self-expansion is the desire to enhance an individual's potential efficacy. Motivational models often refer to self-efficacy as one's belief that they are competent and can achieve specific goals. 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. Some researchers also believe that the motivation for self-expansion is partly rooted in social approval and acceptance.
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 a value-expectancy approach.

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.
Inclusion of the other in the self

A key principle of the 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. "Others" could be interpreted as individuals. Some studies have found that individuals also include groups or communities.
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. 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. 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.
Aron, Aron, Tudor and Nelson conducted several classic studies that scientifically demonstrated that we include the other in the self.

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. 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. 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. Research further shows that such closeness increases overlap between mental representations of self and other, making it harder to differentiate between self‐ and other‐related information, which can impair decision‐making performance and increase cognitive costs in problem‐solving tasks. Such self–other confusion also impairs memory in cooperative decisions: participants made more recall errors when playing trust games with a close partner than with a distant one, especially when they had greater self–other overlap.

Two-dimensional model

A further theory describes relationships changing across two dimensions:

direction
valence

These two dimensions combine to make four possible categories, where self-expansion is defined as an increase in positive qualities.

Name	Direction	Valence	Example
Self-expansion	Increase	Positive	Individual acquires an appreciation for ballet, or becomes a better painter
Self-contraction	Decrease	Positive	Individual neglects friendships which a partner disapproves of
Self-adulteration	Increase	Negative	Individual is influenced by a partner to eat junk food
Self-pruning	Decrease	Negative	Individual quits smoking at the behest of a partner

Both self-expansion and self-pruning are promoted by the Michelangelo phenomenon.

Interpersonal relationships

Initial attraction and relationship maintenance

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. Studies have shown that perceived similarity and likeness can promote interpersonal attraction. People tend to prefer others that are similar to them, which goes against the motivation to increase expansion opportunities. 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.
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. In this case, men were more likely to select dissimilar partners as they provide 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.
For established couples, research shows they can experience different levels of motivation for self-expansion throughout their relationship. 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 who expect future self-expansion through their relationships are more likely to engage in self-expanding activities. Potential for future expansion is a strong predictor of relationship satisfaction and commitment.

Reducing boredom and increasing sexual desire

Research shows that when couples participate in some self-expansion activities, these activities can increase relationship satisfaction and improve relationship quality. Moreover, 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. 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. This follows the 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.
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. 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.