Most cultures have a religion. However, most of those religions don't actually involve a belief in a God (or Gods).
But then farming arose and populations began to grow bigger. In these large populations, belief in a law-giving "King" God became much more common. Since you're likely living in a large population as you read it, these are the sorts of deities your probably most familiar with.
What was it that prompted their appearance? Why did larger populations begin to believe in a moral-giving God?
All (in)equal in the eyes of God
A few years ago researchers identified a very interesting trend in the religious beliefs of different groups. Social size and group complexity was strongly correlated with a belief in a "King" God.
Only ~5% of societies with less than 1,000 people in believed in a law-giving God. A further 20% or so believed in an inactive God. But by far the majority didn't even have a concept of a "deity". Meanwhile more than 40% of groups with >10,000 members believed in such a deity; with an additional 40% also believing in an inactive God.
This same trend was seen with social complexity. Groups with no social stratification were much less likely to have an active deity (or even a deity at all) than groups with wealth distinctions and social classes.
Based on all this many researchers concluded that there was something about larger, more complex groups that necessitated a law-giving deity. One key hypothesis was that such a belief helps foster co-operation and avoid free-riders. Doing so becomes more and more important as a society becomes more and more complex.
More resources need to be produced to sustain it; which requires people working together. But that surplus of resources makes it possible for people to take from the pool without giving back. What better way to encourage people to do the former and avoid the latter than the commandments of a deity?
Additionally, inequality might also be necessary for these societies. They need a ruling class to govern. Nowadays they get their power from the "will of the people", but might the "divine right of kings" be how they first got their authority? Certainly many religious texts preach loyalty to leadership.
Proof its not WEIRD
This hypothesis seems to make sense. However, it is ultimately just a lot of speculation based on general trends seen in society.
So researchers set out to see if belief in a law-giving God actually does encourage co-operation. Multiple studies seem to have proven that this is the case. "Priming" individuals with religious beliefs encourages them to be more generous and work with others in psychological games. In some studies, it almost doubled the amount of money individuals were willing to donate to an anonymous person.
However, most of this research is WEIRD. That is, conducted on Western, Educated, Industrialised, Rich, Democracies. These sorts of societies are an anomaly in human history, so there's debate over whether or not these results can be applied to non-WEIRD societies.
So researchers went out to test just how widespread these trends were. They examined thousands of individuals from dozens of societies spread across every continent (except that one inhabited only by penguins). And sure enough, they confirmed that the results seen in WEIRD societies were actually seen around the world. A belief in a law-giving God was correlated with increased levels of empathy, altruism, compliance, and honesty.
All of the key traits needed to foster co-operation.
Of course correlation is not the same as causation. It could work the other way around, with religion attracting people naturally more co-operative. However, the fact that these trends were so widespread amongst so many different cultures and religions makes this unlikely, in my opinion (although further research to confirm this is always welcome). Breaking down the data further to examine if it was religions with a "law-giving" God could certainly help with this.
Weird outcomes of religion
As well as showing that there was a correlation between religiosity and religion, these researchers also found religion was linked with a few other personality traits.
In particular, higher levels of religiosity were correlated with lower levels of sexual permissiveness. One of the big ways this manifested was less interest in short term mating; with religious people wanting to "settle down". This trend was much more apparent in women than men; except for in people who had never had sex. Religious virgins of both genders were equally less interested in short term mating.
Curiously, this reduced sexual permissiveness was also seen in groups engaging in seemingly "unpermissable" sexual activities. Religious homosexuals and bisexuals also scored low on the scale, similar to their heterosexual counterparts. However, there was less difference between the genders.
Additionally, a negative correlation between religion and openness to new experiences was also found. However, it wasn't as widespread as the other characteristics, being almost absent from Asian populations. In the West though, religion definitely made you less open to new experiences. And slightly less neurotic.
Are these characteristics an evolved part of religion? Do they enable the fostering co-operation? Is it coincidence? The prevalence of these attitudes suggests the latter is untrue, but beyond that there's no clue about why these aspect of religious belief developed.
tl;dr
Belief in a law-giving god is hypothesised to foster co-operation. Cross cultural data confirms this is the case. Also, it shows religious people rank lower on the scale of sexual permissiveness.
References
Peoples HC, & Marlowe FW (2012). Subsistence and the Evolution of Religion. Human nature (Hawthorne, N.Y.) PMID: 22837060
Schmitt, D.P. and Fuller, R.C., 2015. On the varieties of sexual experience: Cross-cultural links between religiosity and human mating strategies. Psychology of Religion and Spirituality, 7(4), p.314.
Shariff AF, & Norenzayan A (2007). God is watching you: priming God concepts increases prosocial behavior in an anonymous economic game. Psychological science, 18 (9), 803-9 PMID: 17760777
