How to stop mayhem?

I was having a discussion/debate with a friend, and he asked

"If you are on an island with 10 people, how do you stop mayhem?" 

If say one person finds some fruit, how do you stop the others from robbing or killing him for it? What about an island with a hundred people, a thousand, or the whole world with 7 billion. His answer was that you need a social contract, or in other words, that you need law.

My answer was totally different. I immediately answered, guns. Give me a gun, and who would rob me for some fruit? Give everyone a gun and there will be peace.

You can all get together and decide on laws, there could be a law stating 'do not steal fruit' or 'do not rob', but does that actually stop people from robbing? ... No it doesn't. It is the punishment for breaking a law that stops people from breaking it. With a gun, who would risk getting shot to steal some fruit from me? And the greatest part is, I don't even need to use my gun. Just the fact that I will use it is enough to stop anyone with half a brain.

However to be more complete there are many pieces required for an effective law:

1. The written rule
2. Detection, a method of telling when a law is broken and who broke it
3. The written punishment
4. Enforcement of that punishment in actuality

Miss any one of these and suffer.

Don't have detection and people will break the law in the shadows. Don't have enforcement and people will break the law in daylight. Have detection and enforcement but not the others, and well, people will obey the unspoken rules, but you will lose your civilness.

No punishment without law is an important principle of any legal system. You can imagine the unjustness of being arrested and jailed without breaking any law. Or a new law being written and you being retroactively punished for breaking it before it existed.

To be even more accurate what's important in preventing people from breaking a law in the first place isn't exactly the punishment, but the percieved punishment. Religion can interestingly work with just the latter and no visible former. Suffer eternal damnation in hell if you don't follow the rules. Quite harsh. Religion is a bit of an exception though, most of the time you still need actual punishment or any perception of punishment will eventually disintegrate. When a few guys break the law and nothing happens to them, others will notice. But enough of that.

To be continued...