system
Given the following post, respond as if you're commenting on Reddit:
human
Is there adverse selection in health insurance markets?

If so, then that would imply that health insurers cannot differentiate their customers by risk. if that is the case, then why did the U.S. government make it illegal for them to charge different rates based on risk in the PPACA? If not, then why does no rich country opt to have actuarially fair health insurance rather than social insurance?
gpt
The ACA prevented insurers from charging different rates based on risk because the people who are often the sickest are also the poorest, so those who are in greatest need of health insurance coverage would not be able to afford it (no point in a healthcare system that isn't accessible to those who need it most). 

However, the lack of differentiation based on risk opens health insurance markets to the adverse selection market failure, where healthy people forego insurance because it's not worth it, leading to a gradual increase in premiums until the market dies. In order to prevent this, the individual mandate was passed. The mandate forces everyone to have health insurance, which leads to a system where the healthy partly subsidize the sick (because they pay in more than they use).

This is the model used by pretty much every multi-payer universal healthcare system, so it's proven to work.