A recent neuroimaging study conducted in Finland revealed that sisters exhibit more similar brain activity than friends or acquaintances when watching a movie. This increased similarity was particularly evident in brain areas associated with complex cognitive functions, including the default-mode network and sensory processing regions. The study, published in NeuroImage, sheds light on the potential neural underpinnings of social bonds, suggesting that the closeness of relationships is reflected in the way our brains process shared experiences.
Homophily refers to the tendency for individuals to form connections with others who are similar to them in various ways, such as beliefs, values, social status, or demographic characteristics. This phenomenon is often observed in social networks, where people with similar backgrounds or interests are more likely to become friends, collaborate, or form communities. While homophily can strengthen social cohesion within groups, it can also contribute to divisions between different groups by limiting exposure to diverse perspectives.
In addition to homophily, people tend to exhibit a preference for family members, known as kinship premium. Kinship premium refers to the stronger bonds and preference for kin over non-kin, stemming from shared ancestry and genotypes. These connections result in relationships that are often more stable and enduring, even across generations and despite physical separation. Family ties, particularly between siblings, are reinforced by shared experiences, upbringing, and emotional closeness.
Previous research has indicated that friends tend to have more similar brain activity patterns than acquaintances when watching the same movie, hinting at a form of neural homophily. However, Mareike Bacha-Trams and her colleagues sought to investigate whether this neural similarity extends to family members, particularly sisters. Specifically, they aimed to determine if the brain activity of sisters is more similar than that of female friends. The researchers also examined whether these similarities could be explained by other factors such as anatomical similarities, eye movements, heart rates, or breathing patterns.
The study involved 30 healthy women aged 19 to 39, organized into 10 triplets. Each triplet consisted of two sisters and a female friend of one of the sisters, who was an acquaintance of the other sister. This setup allowed the researchers to compare brain activity across three types of relationships: sisters, friends, and acquaintances.
Participants watched a 24-minute edited version of the movie My Sister’s Keeper, which presents a moral dilemma involving two sisters: Anna, who is asked to donate a kidney to her terminally ill sister Kate, but refuses. The emotional and ethical complexity of the film made it an ideal stimulus for exploring how different social relationships might influence brain activity.
Each participant underwent four fMRI sessions, watching the movie under different conditions. In each session, participants were instructed to view the movie from the perspective of one of the sisters, combined with information that the sisters in the movie were either genetically related or that one was adopted. After the second session, participants viewed the original ending of the movie, which reveals that the sick sister had asked her healthy sister to refuse the donation. The participants then reported whether this revelation altered their perception of the characters.
Following the fMRI sessions, participants completed various assessments, including questions about the movie, the Hatfield’s Emotional Contagion Scale (which measures emotional empathy), and the BIS/BAS scale (which assesses behavioral inhibition and activation). They also rated their arousal levels during the movie.
The results showed that the brain activity of sisters was more similar than that of friends, and the brain activity of friends was more similar than that of acquaintances. This increased similarity in brain activity was found in widespread areas of the parietal, temporal, and occipital cortices, as well as in parts of the frontal cortex, both laterally and medially. These regions are involved in higher cognitive processes, not just basic sensory perception.
One of the most significant findings was the increased similarity in the default-mode network, a group of brain regions that are active when the mind is at rest and not focused on external stimuli. The default-mode network is associated with self-reflection, daydreaming, and thinking about past or future events. The researchers found that sisters, more than friends or acquaintances, processed and evaluated the movie’s events in a similarly complex and reflective manner.
Interestingly, the similarity in brain activity among sisters could not be attributed to shared eye movements, emotional responses, or physiological factors like heart rate and breathing. This suggests that the resemblance in brain activity is likely due to deeper, possibly intrinsic factors related to their relationship.
“Our findings show that sisters, beyond the simple perception of the stimulus, process and evaluate the events in the movie in a more similar way than subjects with a different relationship. The close resemblance in brain activity between sisters may be a result of common genes, although this possibility is not very likely, as within this study we controlled for the possibility that sisters have higher structural brain similarities. Overall, these results might partly help explain why more cognitive effort is exerted when thinking about friends than when thinking about kin as higher similarity in how one perceives the world makes it less effortful to mentalize about kin than friends, and about friends than about acquaintances.”, study authors concluded.
The study demonstrates that homophily likely has a neural basis. However, it should be noted that the number of study participants was very small, all were women, and they watched only a single movie. Results from studies involving males, different movies, or larger groups of participants might not be identical.
The paper, “Sisterhood predicts similar neural processing of a film,” was authored by Mareike Bacha-Trams, Gökce Ertas Yorulmaz, Enrico Glerean, Elisa Ryyppö, Karoliina Tapani, Eero Virmavirta, Jenni Saaristo, Iiro P. Jääskeläinen, and Mikko Sams.
