Why self-expansion is the key to long-lasting love and friendship

Think back on the great loves of your life and you may remember a heady period when every day seemed to sparkle with new possibilities. It was as if you had been reborn and were looking at yourself and the world around you anew – a dizzying metamorphosis that was as delightful as it was destabilizing.

I’m not just sentimental. A growing body of psychological research shows that our best relationships, whether platonic or romantic, are characterized by an exciting sense of growth. We fall for the people who expand our horizons and help us become a better version of ourselves. And if we want our love to last, we have to work hard to keep nurturing that growth.

This idea—known as self-expansion theory—is the brainchild of Arthur and Elaine Aron, husband-and-wife researchers at Stony Brook University in New York, who first proposed it in 1986. In the mid-1990s, they asked students to answer the open-ended question, “Who are you today?” use as many words or phrases as possible. As you might expect, many of the students began dating during the study period, and when they did, they began to use many more distinct terms in their descriptions: their understanding of themselves had literally expanded, as their partners helped them discover new sides of themselves.

Such changes were also evident in formal psychological questionnaires. The students in the budding relationships began to build greater self-esteem, which reflects feelings of self-worth, and higher self-efficacy, which captures someone’s perceptions of their own abilities.

Research on self-expansion has only accelerated in the last decade, with a number of studies examining the concept in different populations, such as the members of the LGBTQ+ community. In each case, the feelings of personal growth were accompanied by greater passion, commitment, sexual desire for the partner and overall satisfaction in the relationship.

It can even determine a couple’s lifespan together. There are many possible reasons for ending a relationship, but Brent Mattingly of Ursinus College in Pennsylvania and his colleagues have shown that young couples with higher levels of self-expansion at the start of the study were less likely to consider breaking up during the following nine months. Meanwhile, other research has shown that people who experience low levels of self-expansion, on the other hand, are more likely to think about cheating.

The effects of self-expansion can even be seen in the firing of our neurons, according to a study by researchers at Renmin University of China published two years ago. Over a seven-month period, they saw that people who experienced greater self-expansion produced higher levels of synchronization with their partner’s neural activity. Quite literally, they were more likely to be thinking on the same wavelength.

How you and your partner can grow together

Couples can encourage self-expansion in several ways. Through conversations, they introduce each other to new ideas and ways of seeing the world that may not have been visible before, and their mutual encouragement helps each person try challenges that may have seemed unthinkable before. You might never have even considered writing a novel or starting a startup, for example, if your partner hadn’t encouraged you to follow your dreams.

As a couple’s life becomes entangled, they may even come to see themselves as a single entity with pooled resources, so that our partner’s talents—such as creativity—begin to feel like our own. This is known as “inclusion of others in the self” – and it is really another form of self-expansion. If my partner is artistic, I may come to see myself as a bit more creative by mere association; my concerns about climate change, meanwhile, may lead them to become more environmentally conscious. We both would have discovered new sides of ourselves that hadn’t been apparent before.

In the end, there are our shared experiences. As a couple, we can find new hobbies or travel to new places that we had never tried before we met.

Each of these facets can provide tips for igniting feelings of love and for keeping that flame burning long term. We can ensure that our conversations are as deep and meaningful as possible, for example so that we discover more about each other and ourselves. It is no coincidence that Arons also developed the “36 Questions to Fall in Love” that created a viral New York Times column. In the original studies, participants were assigned to strangers and offered a series of discussion points designed to encourage greater self-disclosure, such as:

• Do you have a secret idea about how you want to die?
• What, if anything, is too serious to joke about?
• What is your most valuable memory?

In less than an hour, couples who discuss these topics report greater intimacy than those who engage in milder banter. (You can read more about that here, on the excellent Greater Good in Action website from the University of California, Berkeley.) This may seem harder to do in established relationships, but research suggests that we often overestimate our knowledge of the people closest to us—and we may be surprised by what we discover if we simply take the time to find out what they’re really thinking and feeling.

Second, we can spice up our time together by exploring new places or trying new activities that we’ve never tried before. In a series of studies, Cheryl Harasymchuk of Carleton University in Canada and her colleagues asked people to document their lives with their partners and their feelings towards them. They found that the more exciting or unusual the date nights they had, the greater the sense of self-expansion, and the closer they felt to face value – and the greater the sexual desire for each other. There is no simple recipe. For some it may be wine tasting; for others stargazing or spelunking. Find a new experience that pushes you all outside of your comfort zones.

While research has largely focused on the benefits of self-expansion for relationships, you can also reap some of these benefits while flying solo. In 2024, Emine Yücel at Selçuk University and Duygu Dincer at Istanbul Aydin University, both based in Turkey, showed that self-expansion can enrich our platonic friendships. Some people may even prefer to engage in self-expansion on their own. Expanding your view of yourself and your abilities will be exciting in itself, with or without a partner to encourage you.

Regardless of your relationship status, start opening your mind to new possibilities this Valentine’s Day. If you have a loving partner who can share the experience, so much the better – but you don’t have to wait for “the one” to come along.