Fans of Freakonomics are familiar with some of its wild assertions. The author, economist Steven Levitt, makes correlations between people's behavior and economic and social outcomes. For example, he makes a link between a person's name and financial success.

In biomedical science, epidemiologists and evolutionary biologists use mathematical and statistical methods to link seemingly disparate items to produce some astonishing conclusions. One that has re-surfaced is the link between the establishment of religions and how that may have prevented the spread of infectious diseases.

Before delving into this theory, it's important to know that there are and have been far more religions in tropical regions near the equator than in temperate climates. In Cote d'Ivoire, West Africa, there are 76 distinct and documented religions, compared with Norway, which has 13. In Brazil, another tropical country, there are 159 religions while Canada has 15. These pairs of countries are similar sizes, so what accounts for the large difference in the number of established religions?

Now consider the distribution of infectious diseases. There are far more in tropical than in temperate regions. So the evidence suggests that areas with a greater number of infectious diseases also have greater diversity of practiced religions.

Evolutionary biologists saw this at work in ancient times. For example, major epidemics swept through and decimated populations from 800 to 200 B.C., a time when cities began to flourish. That's also when many religions were established.

One of the explanations for this correlation of religion and infectious disease is that by adopting religions, groups became more insular. This increased cohesiveness and limited exposure to outsiders, lowering the group's chances of contracting infectious diseases.

Multiply the effect through a large population that then breaks into smaller religiously diverse groups, and this may have helped to contain epidemics.

What scientists noticed is over time these groups began to develop and pass on disease-resistant genes, which also limits infection rates. Even in groups where an infectious disease is common, its spread is less likely because people of a religious group tend not to migrate, again, limiting an epidemic.

If this theory is true, scientists would have to consider that another basic purpose of early religion has been to manage and avoid infectious diseases. What's true today is many religions continue to play a central role in controlling infectious diseases by caring for the sick in areas around the world.