Ever since William Shakespeare invented the phrase "band of brothers,” it has been used to describe the deep bonds that form between men who fight together in war. According to one theory of human evolution, the bonds between warriors actually do run as deep as brotherhood, even if they are not kin. This "identity fusion" may have been crucial for our evolution, allowing groups of unrelated humans to cooperate in dire times. Now, a study of the men who took up arms in the Libyan revolution of 2011 finds support for this idea. Right after the war ended, a pair of U.K. researchers surveyed 179 Libyan men from four rebel battalions. Some had been front-line fighters, while others had played nonfighting support roles. The Libyans performed a psychological test called a pictorial fusion scale—a circle representing the self is placed inside or outside circles representing various groups of people—that revealed the depth of their relationships. Forty-five percent of front-line fighters identified more closely with each other than they did with their own families, compared with 28% of noncombat personnel, the team reports online today in the Proceedings of the National Academy of Sciences. But when it came to identifying with the rest of the Libyan citizens who supported the revolution but did not join a battalion, only 1% of rebels felt any identification. Not only does it show that a "band of brothers" is an accurate description of fellow fighters, but the lack of solidarity of the fighters with ordinary citizens may be partly to blame for the bitter infighting that has emerged in Libya and elsewhere in the wake of successful revolutions.