Falling in love is not just about butterflies in your stomach: chemical fireworks are lit. The reward center, nourished by dopamine, activates as if faced with a drug. For years, neuroscientists have been trying to understand how this
love brain distinguishes the chosen one from a simple friend. A study published in 2025 in the journal Social Cognitive and Affective Neuroscienceconducted at Kyoto University, sheds light on this question. What they observe challenges some preconceived ideas.
The management of love by the brain’s reward center
Romantic relationships encompass complex psychological states that differentiate a romantic partner from a platonic friend. Researchers have attempted to map these distinctions in the brain to better understand the biological basis of human emotional bonds. A significant part of these studies focus on the nucleus accumbens. Within the brain, the nucleus accumbens participates in the “reward circuit”, which means that it is activated when the subject expects a reward. In this case, it receives activating dopaminergic signals (serotonin is rather inhibitory).
In the prairie vole, a monogamous rodent, work has shown that this zone transforms to promote preference for a single partner and maintain the bond within the pair. But research on humans has so far been unable to find the same changes.
“Our goal was to identify the neural mechanisms that distinguish romantic partners from friends. Additionally, as romantic relationships develop, most individuals experience psychological fluctuations, raising the question of how a partner’s neural processing may change with relationship maturity. Finally, given previous theories and data suggesting that opposite-sex friends can sometimes be potential or alternative partners, we were interested in whether the brain represents an opposite-sex friend more similarly to a romantic partner or a same-sex friend.” declares in PsyPost, Kenji Fujisaki, author of the study and member of the psychology department at Kyoto University.
At the beginning, the love brain overinvests in the partner
In 47 young men who had been in a relationship for an average of 18 months, the researchers linked the activity of the nucleus accumbensa key region of pleasure, to the duration of the relationship, by comparing very recent stories and others that are more established.
Each participant came with a romantic partner, a friend of the opposite sex and a friend. These friends, along with their romantic partners, provided short video clips for the experiment. Lying in the MRI, he had to quickly press a button to receive a video where one of his loved ones smiled and made a positive gesture, like a V for victory. The volunteers responded more quickly and judged their partner’s videos more enjoyable than those of their friends. If participants were too slow, they saw a neutral expression. This device allowed researchers to isolate brain activity associated with the anticipation of social approval from specific people.
Beyond the intensity of the signal, the team analyzed the precise shape of the activity patterns, a kind of neural imprint specific to each person.
Over time, the partner’s neural signature fades
At the beginning of the relationship, the nucleus accumbens encodes the partner in a specific way, different from that of the friend of the opposite sex, who resembles the male friend. Then this specificity decreases with the duration of the couple. “Our findings suggest that the way the brain represents a romantic partner is not fixed, but can evolve as a relationship develops,” Kenji Fujisaki told PsyPost.Early in relationships, a reward-related brain region called the nucleus accumbens showed clearly differentiated activity patterns for a partner versus an opposite-sex friend. In longer relationships, this neural distinction became less pronounced. This change may reflect a shift from the passionate love characteristic of early relationships to a more stable, companionate form of love that shares characteristics with deep friendship“.
The longer the relationship lasted, the less the nucleus accumbens distinguished the partner from the friend. This link persisted after accounting for intimacy, passion, and commitment scores. For researchers, this is a biological shift, not a loss of feeling.
A less passionate love, but supported by other brain circuits
All this ties in with the idea of a very dopaminergic brain at the beginning, then of a peaceful attachment where the partner becomes a close friend.
As with any study, this one has certain limitations. It juxtaposes individuals at various stages of their relationship instead of following the same people over time; furthermore, the sample consisted of heterosexual men only. Future research will need to incorporate more diverse samples to test whether these neural models are universal.