What happens in the case of an intentional hard block chain fork?

[tlr]

(Sorry if this has been discussed before, I couldn't find it in the search)

I understand that if a minority group intentionally or unintentionally forks from the main chain their transactions won't be recognized by the vast majority of users and their chain will be pretty much worthless and die.

But what if some large portion of the Bitcoin "economy" decides to fork, and their fork actually survives? How would you calculate the effect on the value of the coins in each fork? Some users might only accept transactions on one chain or the other, but some might accept both. If it's an even split, does each fork lose half of it's "real" value?

[mgio]

It can't just be a large portion, it has to be the majority of the miners out there. In other words >50% of the mining power out there has to agree to the fork. The only way this could happen is if someone with a ton of money (like the US government) dedicated resources to building machines just for mining. And even then, the advantages to forking the blockchain aren't really that great. You can double spend some bitcoins but you can't actually steal them.

[tlr]

It can't just be a large portion, it has to be the majority of the miners out there. In other words >50% of the mining power out there has to agree to the fork. The only way this could happen is if someone with a ton of money (like the US government) dedicated resources to building machines just for mining. And even then, the advantages to forking the blockchain aren't really that great. You can double spend some bitcoins but you can't actually steal them.

It's a hypothetical question. I'm not talking about a 51% attack.

Let's say exactly 50% of the economy (*not* necessarily 50% of miners, see https://en.bitcoin.it/wiki/Economic_majority) decide for whatever reason (philosophical differences or whatever) to adopt one set of rules while the other 50% decides to keep the old set of rules.

What happens?