Why do people so readily believe in Gods?  Dominic D.P. Johnson presents “Error Management Theory” (the name plays off Terror Management Theory).  This is an excerpt from the abstract Johnson's book, The Error of God: Error Management Theory, Religion, and the Evolution of Cooperation:

“Error Management Theory” is derived from signaling theory, suggests that if the costs of false positive and false negative decision-making errors have been asymmetric over human evolutionary history, then natural selection would favor a bias towards the least costly error over time (in order to avoid whichever was the worse error). So, for example, we have a bias to sometimes think that sticks are snakes (which is harmless), but never that snakes are sticks (which may be deadly). Applied to religious beliefs and behaviors, I derive the hypothesis from EMT that humans may gain a fitness advantage from a bias in which they tend to assume that their every move (and thought) is being watched, judged, and potentially punished by supernatural agents. Although such a belief would be costly because it constrains freedom of action and self-interested behaviors, it may nevertheless be favored by natural selection if it helps to avoid an error that is even worse: committing selfish actions or violations of social norms when there is a high probability of real-world detection and punishment by victims or other group members. Simply put, supernatural beliefs may have been an effective mindguard against excessively selfish behaviour – behavior that became especially risky and costly as our social world became increasingly transparent due to the evolution of language and theory of mind. If belief in God is an error, it may at least be an adaptive one.

Religious believers incur significant costs in terms of time, energy and resources that could be spent elsewhere. Religion therefore poses a major puzzle for disciplines that explain behavior on the basis of individual costs and benefits—in particular economics and evolutionary biology. To many scholars, religious beliefs and behaviors appear so bizarre and so costly that they fall outside rational explanation, leading instead to explanations based on psychosis, cognitive accidents, or cultural parasites. The aim of our project is to conduct a scientific examination of exactly the opposite hypothesis—that religious beliefs and behavior confer adaptive advantages to individual believers, and were therefore favored by natural selection over human evolutionary history. In other words, religion may have evolved.