The science behind why birds of a feather flock together goes beyond just sharing mutual interests, according to a recent study published by Nature Communications.  

According to UCLA and Dartmouth researchers, friends perceive, interpret and respond to the world in a manner that isn't exclusive to similarities in personality, age, gender, ethnicity or other demographic factors. Simply put, friends are cognitively homophilous, or share similar brain wavelengths.  

"Similarity in how individuals interpret and respond to their environment increases the predictability of one another's thoughts and actions during social interactions, since knowledge about oneself is a more valid source of information about similar others than about dissimilar others," said lead author Carolyn Parkinson, PhD, an assistant professor of social psychology at UCLA.  "This increased predictability during social interactions, in turn, allows for less effortful and more confident communication, thus fostering more enjoyable social interactions and increasing the likelihood of developing friendships."

According to study methods, 279 students from an unspecified graduate program completed an online survey. All participants were required to indicate other study participants with whom they were friends with.  

In addition, a subset of 42 participants watched video clips while undergoing fMRI. Participants were told prior to being scanned that the video would vary in content and would switch consistently from one clip to another, though all were shown in the same order to the participants.

The main objective for this portion of the study was to see whether similarities of neural responses can be used to predict the "social distance" between participants. 

"Differences in the similarities of subjects’ neural response time courses likely stem from factors such as differences in subjects’ dispositions, moods, cognitive styles, pre-existing assumptions, expectations, values, views and interests, as well as differences in the pre-existing knowledge structures into which incoming stimuli are integrated," Parkinson and colleagues wrote.  

Researchers noticed that nationality and gender contributed greatly to social difference. Further, logistic regression analysis analogues revealed that the ventral and dorsal striatum regions of the brain (areas responsible for memory, language processing, learning and motivation) drive the relationship between social distance and overall neural similarity between individuals who are friends and who are not. 

"Logistic regressions that combined all non-friends into a single category, regardless of social distance, yielded similar results, such that neural similarity was associated with a dramatically increased likelihood of friendship, even after accounting for similarities in observed demographic variables." Parkinson et al. said. 

Neural responses tracked by fMRI examinations on the 42 participants were more greatly similar among friends than among individuals who were not. The developed predictive dyad pattern models trained to differentiate social distance based on patterns of inter-subject neural response similarity also accurately predicted the friend status and "social distance" of pairs of individuals, according to study results.

"The current findings extend this research by demonstrating that covert mental responses to the environment, as indexed by neural processes evoked naturalistically during undirected viewing of videos, are exceptionally similar among friends," Parkinson et al. added. 

The study contained a few notable limitations, including the dismissal to compare study results to other similar studies driven by behavioral measures analyzed in the brain. According to the researchers, comparable results would not have been possible without neuroimaging. 

"Future studies may wish to adopt experimental designs that allow for drawing inferences about exactly what kinds of stimuli are particularly important for predicting patterns of real-world social ties," Parkinson said. 

The researchers asked: Do we become friends with people who respond to the environment similarly, or do we come to respond to the world similarly to our friends? 

"Although the results of the current study suggest that friends have exceptionally similar neural responses to naturalistic stimuli, due to this study’s cross-sectional nature, we cannot ascertain, based on these results alone, whether neural response similarity is a cause or consequence of friendship," Parkinson et al. concluded. "Thus, future longitudinal studies should measure whether inter-subject neural response similarities predict subsequent friendship formation among members of evolving social networks."