One proposed explanation for the ubiquity of religious belief in human societies is multi-level selection: altruists often lose out within groups, but groups with more altruists win. A group whose members believe a "practical reality" that motivates altruistic behavior may out-compete other groups whose members' perception of "factual reality" causes them to behave selfishly. A classic example is a soldier's willingness to fight for his tribe or country. Another example given in evolutionary biologist David Sloan Wilson's "Darwin's Cathedral" is the early Christian church under the late Roman Empire; because Roman society was highly individualistic, during frequent epidemics many of the sick died not of the diseases "per se" but for lack of basic nursing. Christians, who believed in an afterlife, were willing to nurse the sick despite the risks. Although the death rate among the nurses was high, the average Christian had a "much better chance" of surviving an epidemic than other Romans did, and the community prospered.