If you were asked to fall backward into the arms of a stranger, would you trust the other person to catch you? Such a situation, a common exercise in group therapy, is a bit extreme. But every day most people place some degree of trust in individuals they do not know. Unlike other mammals, we humans tend to spend a great deal of time around others who are unfamiliar. Those who live in cities, for instance, regularly navigate through a sea of strangers, deciding to avoid certain individuals but feeling secure that others will, say, give accurate directions to some destination or will, at the very least, refrain from attacking them.