The light and dark sides of friendship.

I want to point to two brief reviews by Natalie Angier on friendship. She first points to work of Parkinson et al. showing similarities in the brain activities of friends. The Parkinson et al. abstract:

Human social networks are overwhelmingly homophilous: individuals tend to befriend others who are similar to them in terms of a range of physical attributes (e.g., age, gender). Do similarities among friends reflect deeper similarities in how we perceive, interpret, and respond to the world? To test whether friendship, and more generally, social network proximity, is associated with increased similarity of real-time mental responding, we used functional magnetic resonance imaging to scan subjects’ brains during free viewing of naturalistic movies. Here we show evidence for neural homophily: neural responses when viewing audiovisual movies are exceptionally similar among friends, and that similarity decreases with increasing distance in a real-world social network. These results suggest that we are exceptionally similar to our friends in how we perceive and respond to the world around us, which has implications for interpersonal influence and attraction.

Angier also notes work showing that the other side of homophily, or friendship, can be the urge to "otherize" those who differ from you and your friends.

...the study from the University of Michigan had subjects stand outside on a cold winter day and read a brief story about a hiker who was described as either a “left-wing, pro-gay-rights Democrat” or a “right-wing, anti-gay-rights Republican.” When asked whether the hypothetical hiker might feel chilly as well, participants were far more likely to say yes if the protagonist’s political affiliation agreed with their own. But a political adversary — does that person even have skin, let alone a working set of thermal sensors?

The abstract of that study:

What people feel shapes their perceptions of others. In the studies reported here, we examined the assimilative influence of visceral states on social judgment. Replicating prior research, we found that participants who were outside during winter overestimated the extent to which other people were bothered by cold (Study 1), and participants who ate salty snacks without water thought other people were overly bothered by thirst (Study 2). However, in both studies, this effect evaporated when participants believed that the other people under consideration held political views opposing their own. Participants who judged these dissimilar others were unaffected by their own strong visceral-drive states, a finding that highlights the power of dissimilarity in social judgment. Dissimilarity may thus represent a boundary condition for embodied cognition and inhibit an empathic understanding of shared out-group pain. Our findings reveal the need for a better understanding of how people’s internal experiences influence their perceptions of the feelings and experiences of those who may hold values different from their own.