A new study published in the journal Social Cognitive and Affective Neuroscience provides evidence that the human brain processes romantic partners differently than close friends, specifically within the reward system. The research suggests that while the brain creates a unique neural signature for a partner early in a relationship, this distinction tends to fade as the bond matures. These findings offer insight into how the biological drivers of romantic love may evolve from passion to companionship over time.
Relationships involve complex psychological states that differentiate a committed partner from a platonic friend. Scientists have sought to map these differences in the brain to understand the biological foundations of human bonding. Much of this research focuses on the nucleus accumbens. This small region deep within the brain, which relies heavily on the neurotransmitter dopamine, plays a central role in processing rewards and motivation.
Evidence from animal studies indicates that the nucleus accumbens is essential for forming pair bonds. Research on monogamous prairie voles shows that neurochemical signaling in this area drives the preference for a specific partner. The brain appears to undergo plastic changes that reinforce the bond.
Human studies have attempted to replicate these findings by comparing brain activity in response to partners versus friends. However, the results have been inconsistent. Some experiments observed higher activity in the nucleus accumbens for partners, while others found no significant difference. This inconsistency might stem from the fact that opposite-sex friends can sometimes be viewed as potential romantic alternatives.
“Romantic relationships are typically characterized by exclusivity, strong commitment, and passionate love, which distinguish them from friendships,” said study author Kenji Fujisaki
of the Department of Psychology at Kyoto University.
“We aimed to identify the neural mechanisms that distinguish romantic partners from friends. In addition, as romantic relationships develop, most people experience psychological fluctuations over time, raising the question of how neural processing of a partner may change as a relationship matures. Finally, given prior theory and evidence 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 to a same-sex friend.”
The study involved 47 heterosexual male participants. All participants were between the ages of 20 and 29 and were currently in a romantic relationship. The average length of these relationships was approximately 18 months. The researchers excluded individuals who were married or had children to control for the effects of long-term domestic partnership or parenthood.
To ensure the study captured genuine social bonds, the participants selected their own close friends to be part of the stimuli. They chose a close female friend and a close male friend. These friends, along with the romantic partners, provided short video clips for the experiment.
The researchers used functional magnetic resonance imaging to monitor brain activity while participants engaged in a specific activity called the social incentive delay task. This task is designed to measure the anticipation of a social reward. Participants saw a cue on a screen indicating which person would appear.
After a short delay, a target appeared on the screen for a fraction of a second. Participants had to press a button as quickly as possible. If they were successful, they saw a video clip of their partner, female friend, or male friend smiling and making a positive gesture. These gestures included waving, clapping, or making a “V-sign.”
If the participants were too slow, they saw a neutral expression instead. This design allowed the researchers to isolate the brain activity associated with anticipating social approval from specific people. The team analyzed the imaging data using a technique known as multivoxel pattern analysis.
Standard analysis looks at whether a brain region is “on” or “off.” In contrast, multivoxel pattern analysis examines the specific pattern of activity across many small segments of brain tissue. This allows researchers to see if the “neural fingerprint” for one person differs from another, even if the overall activity level is the same.
The behavioral results showed that the men were highly motivated by their partners. Participants reacted faster when anticipating a video of their partner compared to either friend. They also rated the videos of their partners as more likeable than those of their friends.
The brain imaging results revealed that the nucleus accumbens encodes the romantic partner in a distinct manner. The computer algorithms used in the analysis successfully differentiated the brain activity patterns associated with the partner from those associated with the female friend. This discrimination was possible across the nucleus accumbens and other related brain structures.
The researchers then assessed the similarity of these neural patterns. They found that in the nucleus accumbens, the representation of the female friend was more similar to the male friend than to the partner. This suggests that the brain categorizes the partner as a unique social entity, distinct from the general category of friendship.
A key finding emerged regarding the duration of the romantic relationships. The researchers analyzed whether the distinctiveness of the partner’s neural signature was related to how long the couple had been together. They observed a negative correlation between relationship length and neural specificity.
Participants who had been in their relationships for a longer time showed less distinct neural differences between their partner and their female friend. In the nucleus accumbens, the unique pattern that separated the partner from the friend appeared to diminish as the relationship length increased. This trend remained statistically significant even after the researchers controlled for self-reported levels of intimacy, passion, and commitment.
These results align with psychological theories describing the trajectory of love. Early stages of romance are often characterized by “passionate love,” which involves intense longing and motivation. This stage likely requires highly specific activity within the brain’s reward system to facilitate bond formation.
As a relationship stabilizes, it often transitions into “companionate love.” This form of love is characterized by deep attachment and friendship. The findings suggest that as this transition occurs, the biological processing of the partner in the reward system becomes less distinguishable from that of a close friend.
This reduction in neural distinctiveness does not imply a decline in the quality of the relationship. It may instead reflect a shift in how the relationship is biologically maintained. The intense, reward-driven signaling required to establish a bond may be less necessary for maintaining a stable, long-term union.
“Our results suggest that the way the brain represents a romantic partner is not fixed, but can evolve as a relationship develops,” Fujisaki told PsyPost. “Early in relationships, a reward-related brain region called the nucleus accumbens showed clearly differentiated activity patterns for a partner compared with 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-stage relationships toward a more stable, companionate form of love that shares features with close friendship.”
As with all research, there are some limitations. The research relied on cross-sectional data. This means it compared different people at different relationship stages rather than following the same individuals over time. Longitudinal studies would be necessary to confirm that these changes occur within the same person.
The sample consisted entirely of heterosexual males. This decision was made to reduce biological variability in the sample. However, it limits the ability to generalize the findings to women or individuals with different sexual orientations. Future research needs to include more diverse samples to see if these neural patterns are universal.
The study focused primarily on the nucleus accumbens and the dorsal striatum. While these areas are central to reward, other brain regions are involved in social bonding. Areas responsible for emotional regulation or cognitive processing may take on a larger role in long-term relationships.
There is also the potential for misinterpretation regarding the “reduced specificity” finding. “A common misinterpretation would be to assume that reduced neural distinctiveness means that love or relationship quality is declining,” Fujisaki said. “Our findings do not support this conclusion, and the observed pattern should be understood as a group-level tendency that may vary across individuals.”
Future research could investigate identifying these complementary brain systems. It would be valuable to understand what neural mechanisms support enduring bonds once the specific reward processing in the nucleus accumbens becomes less pronounced. Additionally, examining how major life transitions like cohabitation or marriage affect these patterns could provide further insight.
“This study raises a new question: if partner-specific processing in the nucleus accumbens becomes less distinct over time, what neural mechanisms help sustain long-term relationships?” Fujisaki explained. “Moving forward, it would be worth identifying complementary brain systems that support enduring bonds.”
“In addition, further developing this work by examining neural processes underlying cognition and behavior characteristic of romantic relationships, while taking individual differences into account, may deepen our understanding of romantic bonding. Ultimately, this line of research could provide insights that help foster healthier and more satisfying romantic relationships.”
The study, “Reduced neural specificity for a romantic partner in the nucleus accumbens over relationship duration,” was authored by Kenji Fujisaki, Ryuhei Ueda, Ryusuke Nakai, and Nobuhito Abe.
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