Several theories have been proposed for why religions and religious beliefs have evolved, but before now none of them have involved parasites.

Previous theories have suggested that religions help enforce group cooperation. Another suggestion is that religious thoughts and practices are a side-effect of mental abilities that have evolved for other purposes. For example, prayer is a small step from our evolved ability to rehearse what we plan to say to someone who isn't physically with us right now.

Crucially, none of these accounts can readily explain why the diversity of religions varies so much around the world. Brazil, for example, has 159 religions compared with Canada's 15, even though both countries are of comparable size.

Now Corey Fincher and Randy Thornhill have tested the idea that religious diversity is a side-effect of the fragmentation of cultures that tends to occur in the face of increased threat from infectious disease.

Fincher and Thornhill used the World Christian Encyclopedia and the Global Infectious Disease and Epidemiology Network to compare the spread of infections and religions across 219 countries. Their results were clear: in regions with a greater variety of infectious parasites, the diversity of religions also tends to be greater. This association held strong even after exploring the impact of other potential factors, such as differences in democratisation and histories of colonisation.

The researchers say the association between religion and parasites occurs because reducing contact with outsiders can help protect against disease. In turn, when cultures fragment and groups avoid making contact with each other, more religions are likely to spring up.

"Although religion apparently is for establishing a social marker of group alliance and allegiance, at the most fundamental level, it may be for the avoidance and management of infectious disease," Fincher and Thornhill said. The pair also believe that the diversity of languages and parasites tends to co-vary across the globe for similar reasons.

一些探索宗教和宗教信仰的演變原因的理論已經有人提出了，但到現在為止還沒有任何相關理論涉及寄生蟲。

以前的理論認為，宗教促進人類的團體合作。也有的認為，宗教思想和實踐是人類用于其它目的所形成的心智能力的一種副作用。舉例來說，祈禱來源于人們排練對不在身旁的人說話的內容。

最重要的是，所有這些理論都不能解釋世界上宗教多樣性的原因。例如，巴西有159種類型的宗教，而加拿大卻只有15種，即使這兩個國家的面積大小相關不大。

現在科里‧芬奇和蘭蒂‧陶希爾已經證實他們的想法：宗教的多樣性是文明分散的一種副效應。而文明分散往往是因為面臨來自傳染病不斷擴大的威脅產生的。

利用世界基督教百科全書、全球傳染病和流行病學網絡，芬奇和陶希爾在219個國家內對傳染病傳播與宗教傳播進行比較。其結果是明確的：在傳染性寄生蟲種類繁多的區域，宗教的多樣性也往往會更明顯。這種聯系對之后探討其他潛在因素有很大的影響，如不同的民主化歷程和不同的殖民統治歷史。

研究人員表示，宗教和寄生蟲之間的關聯是因為減少與外界接觸可以幫助抵御疾病。反過來，當文明的片段或人類群體避免彼此接觸，更多的宗教就有可能涌現。

“雖然宗教表面是為團體同盟或忠誠建立一種社會標志，但根本上，它可能是為了避免和控制傳染病， ”芬奇和陶希爾說。兩人還認為，語言和寄生蟲在世界各地的多樣性可能出于同樣的原因。