I'd suggest what keeps man from eating man, on the scale that lions eat zebras, is a general belief in a authority higher than man.
http://www.thefreedictionary.com/argument
My statement was not meant to be an argument or statement of fact but rather a suggestion or hypothesis, which is why I used the word suggest.
I don't think your claim withstands any scrutiny.
If you were able to control for all other conditions, possibly but not necessarily. Thus I don't think comparing homicide rates in the USA, Pakistan and Sweden proves or disproves anything.
So, comparing homicide rates across countries disproves that more religion is necessary or sufficient for an ethical life.
(Let me note in passing that fairly secular northern Europe also has much lower rates of abortions, STDs, teenage pregnancies, and other societal ills, than other, more religious societies.)
Second, if we look at long term trends, violence has come down over the millennia, as religiosity has decreased (see Pinker's Better Angels).
Third, I concede that under certain circumstances in primitive societies you might achieve a reduction in crime by introducing certain religious beliefs (whether they're falsehoods or not). However, we should not strive for that, but for enlightened societies where that is not necessary anymore.
I think you'll be surprised by the amount of cannibalism in the animal kingdom:
Not really the point I was making with the lion/zebra thing, but regardless:
>is a general belief in a authority higher than man.
For some, certainly. I'd venture that it's only one route available. I certainly don't believe in any higher authority whatsoever nor do I even believe that good & evil are particularly sound concepts (Moral Relativism), yet most people that know me will probably judge me a good person.
The writings of C.S. Lewis crystallized a lot of this in my mind.
I'm an atheist, but I certainly have no desire or intention or plan to cease cooperation. Just the opposite, I think it's extremely important, and it is also extremely important that we cherish and enforce and cultivate it.
As just a little example, for me some sort of communal health insurance covering everyone is a matter of course, as for most citizens of developed nations. (I'm aghast at the fact that many people oppose it in the USA, and I note in passing that it appears to be mostly religious people.)
Next, I have the same expectation of basically everyone I encounter (except some sociopaths, against one has to guard appropriately).
Finally, I think there are many good reasons for it, and the position that man is ("just") a higher evolved animal is not at all incompatible with it.
No I'm not saying a belief in God is necessary for what we would agree upon as moral behavior. However, I could easily see the case being made by individuals who might think that what we agree on as morally correct behavior being nothing more than our arbitrary beliefs. And that, from their perspective, rape or pedophilia makes them feel good in the way eating red meat makes them feel good.
So on a spectrum with complete moral bliss on the far left and complete degeneracy on the far right, I think we live somewhere on the left side. Furthermore I think a big reason we live there and not right of center is the wide spread belief in a higher authority/authorities.
> Next, I have the same expectation of basically everyone I encounter (except some sociopaths, against one has to guard appropriately).
Why? From where is your sense of "Fair Play" derived? Does it all boil down to the non-aggression principle and if so, from where is that derived?
Interesting. I am also concerned by moral arbitrariness and relativism, but fear it comes from somewhere else:
I fear that religion opens up the door to moral relativism, because you can go and say "don't eat shrimp" (Leviticus 11:10), or "don't eat pork", or "don't eat cows", or what have you, claiming "it says so right here, in this holy book full of true revelations".
That's a trivial example, but the same applies for more serious issues: morality based on (religious) revelation is hard to verify and discuss, and somewhat arbitrary.
But if we empirically agree on one real world, empiricism and evidence, we have at least a common basis and need very few, very basic moral principles (it really boils down to the golden rule, or somewhat more sophisticated versions of that, like the "veil of ignorance" in John Rawl's Theory of Justice.)
So, in purely practical terms, I think this notion of some psychopath running around claiming "there's no God, thus I can do what I want" is a non-issue.
If you furthermore seek a philosophical "Letztbegründung" (i.e. ultimate reason, where "the buck stops"), there are attempts e.g. in discourse ethics ("if we are talking, we are already implicitly subscribing to certain axioms, otherwise we would not need to talk"). But I don't think it'a a big problem.
The problem lies less in the foundation and more in certain topics, I think (think of abortion, contraception, war).
I think you'd find increased cooperation within many other types of groups. What we should aim for, of course, is to expand this circle of concern ever further, beyond religion.
So, just want to clarify that this might have been an benefit of religion in earlier days, but is by no means an argument for them today.