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

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

I’m not just being sentimental. A growing body of psychological studies shows that our best relationships, be they platonic or romantic, are characterised by an exhilarating feeling of growth. We fall for the people who expand our horizons and help us to become a better version of ourselves. And if we want our love to last, we must 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?” using as many words or phrases as possible. As you might expect, many of the students started dating during the study period, and when that was the case, they started to use many more distinct terms in their descriptions: their understanding of themselves had literally expanded, as their partners helped them to discover new aspects of themselves.

Such changes were also apparent in formal psychological questionnaires. The students in the budding relationships started 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 into self-expansion has only accelerated in the past decade, with a spate of studies examining the concept in diverse 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 their partner and overall relationship satisfaction.

It may even determine a couple’s longevity together. There are many possible reasons for ending a relationship, but Brent Mattingly at 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 over the following nine months. Meanwhile, other research has shown that people who experience low levels of self-expansion, in contrast, 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 synchrony 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 multiple ways. Through conversation, they introduce each other to new ideas and ways of looking at the world which may not have been apparent before, and their mutual encouragement helps each person to attempt challenges that may have seemed unthinkable before. You might have never even considered writing a novel or founding a start-up, for instance, if your partner hadn’t urged you to follow your dreams.

As a couple’s lives become enmeshed, 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 as if they are our own. This is known as the “inclusion of other in the self” – and it’s really another form of self-expansion. If my partner is artistic, I might come to see myself as a bit more creative through sheer association; my concerns about climate change, meanwhile, could lead them to become more environmentally aware. We would have both discovered new aspects of ourselves that had not been evident before.

Finally, there are our shared experiences. As a couple, we may pick up new hobbies or travel to new places that we’d never attempted before we met.

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

• Do you have a secret hunch about how you will die? 
• What, if anything, is too serious to be joked about?
• What is your most treasured memory?

In less than an hour, couples discussing these themes report greater intimacy than those engaging in blander small talk. (You can read more about it here, at 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 we often overestimate our knowledge about 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 are really thinking and feeling.

Secondly, we can spice up our time together by exploring new places or attempting novel activities that we have never tried before. In a series of studies, Cheryl Harasymchuk at 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 their date nights, the greater the feelings of self-expansion, and the closer they felt to their paramour – and the greater their sexual desire for each other. There is no simple prescription. For some it may be wine-tasting; for others stargazing or spelunking. Find a new experience that pushes you each outside of your comfort zones.

While the research has largely focused on the benefits of self-expansion for relationships, you can also glean 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. Enlarging your view of yourself and your capabilities will be exhilarating in its own right, with or without a partner to encourage you.

No matter what your relationship status, you could start to open your mind to new opportunities 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 get started.