25%attack

As we saw, to mine a block, a bunch of miners calculate millions hash per second until finding one with some characteristics. Then they earn a reward for their mined block and they of course share among them the reward accordingly to their contribution of hash rate power.

Because gains are proportional to the hash rate power contributed, there is no incentive for miners to concentrate their efforts on one block. In other terms there is no incentive for always bigger pool of miners which could lead to a majority one.

Example: you own the power to calculate 10 billion hash/day, you can try to mine alone in order to earn the reward alone. Or, you can ally with one or more other miners to find a block faster than alone, but then you will have to share the reward with them. So, mining alone or in group: there are no incentive, at the end of the day you only earn for what you worked (calculate), there are no “economy of scale”.

This proportionality of earnings is the key for the security of the system. As there is no incentive for miners to cooperate to ever growing pools, the blockchain is not exposed to a 51% attack.
Here we will see that it exists a mining strategy which consequence is the incentive for growing pools.

Assume some dishonest miners want to earn more than they deserve by mining honestly. As we know, mining blocks is something quite heavy nowadays, and it needs a lot of power to be able to mine a block. So, these dishonest miners have to own a significant power in their pool, let’s say 25% of the total hash rate power existing (which is far from being the majority).