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. 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.