Here’s something I happened to be reading just now regarding the rapid diffusion of group-beneficial cultural traits:

The rapid spread of Christianity in the Roman Empire may provide an example of this process. Between the death of Christ and the rule of the emperor Constantine, a period of about 260 years, the number of Christians increased from only a handful to somewhere between six million and thirty million people (depending on whose estimate you accept). This sounds like a huge increase, but it turns out that it is the equivalent of a 3%-4% annual rise, about the growth rate of the Mormon Church over the last century. According to sociologist Rodney Stark [The Rise of Christianity, 1997], many Romans converted to Christianity because they were attracted to what they saw as a better quality of life. In pagan society the poor and sick often went without any help at all. In contrast, in the Christian community charity and mutual aid created ‘a miniature welfare state in an empire which for the most part lacked social services.’

Such mutual aid was particularly important during the epidemics that struck the Roman Empire during the late imperial period. Unafflicted pagan Romans refused to help the sick or bury the dead, sometimes leading to anarchy. In Christian communities, strong norms of mutual aid produced solicitous care of the sick, thereby reducing mortality. Both Christian and pagan commentators attribute many conversions to the appeal of such aid. For example, the emperor Julian (who detested Christians) wrote in a letter to one of his priests that pagans need to emulate the virtuous example of the Christians if they wanted to compete for their souls, citing ‘their moral character even if pretended’ and ‘their benevolence toward strangers.’ Middle-class women were particularly likely to convert to Christianity, probably because they had higher status and greater marital security within the Christian community. Roman norms allowed concubinage, and married men freely engaged in extramarital affairs. In contrast, Christian norms required faithful monogamy. Pagan widows were required to remarry, and when they did they lost control of all their property. Christian widows could retain property, or, if poor, would be sustained by the church community.

- Richerson and Boyd, Not by Genes Alone: How Culture Transformed Human Evolution, 2005, p. 210.

Does this sort of cultural analysis offer insights into why Christianity is more successful in some parts of the contemporary world than in others? Or do these pragmatic naturalistic considerations apply to Christendom rather than to Christianity?