Most people believe that their premiums are tailored to them. In reality, they are usually based on average health care costs across the entire risk pool. A large risk pool allows the higher costs of less healthy individuals to be offset by the lower cost of healthier individuals, which can keep premiums affordable and stable. However, if a pool contains too many unhealthy individuals, the premiums can become unaffordable and even lead to adverse selection, where healthy individuals stop buying coverage altogether.