As enrollment statistics in the new health insurance marketplaces start to become available, there is a growing focus on whether the enrollment of so-called “young invincibles” will be sufficient to keep insurance markets stable according to the Kaiser Family Foundation excerpted below.

Why does the age distribution of enrollees matter?

The Affordable Care Act (ACA) requires insurers in the individual market to cover anyone who wishes to enroll and restricts how insurers can vary premiums based on enrollee characteristics. Premiums cannot vary at all based on health status or gender. Premium variations based on age are limited to a ratio of three to one (meaning the premiums for a 64 year-old is three times the premium for a 21 year-old). Previously, premium variations based on age were more typically about five to one.

The limit on age rating means that, on average, older adults will be paying premiums that do not fully cover their expected medical expenses, while younger adults will be paying premiums that more than cover their expenses. For this system to work, young people need to enroll in sufficient numbers to produce a surplus in premium revenues that can be used to cross-subsidize the deficit created by the enrollment of older people. If that does not occur, premium revenues will fall short of expenses and insurers may seek to raise premiums the following year.

While enrollment in the federal and state-based marketplaces have tended to receive the most attention – and are the only enrollment statistics currently being reported – it is the age distribution across the entire individual market that matters from the perspective of the risk pool. That is because insurers are required to set premiums based on a “single risk pool” that encompasses all plans newly-purchased or renewed after January 1, 2014, both inside and outside the marketplaces.

What happens if enrollment among young adults falls short?

Because young adults will be cross-subsidizing older adults, they need to enroll in sufficient numbers for that cross-subsidy to be sufficient. If enrollment among young adults falls short, then the total amount of premiums collected by insurers will be less than the total health care expenses of enrollees plus administrative overhead and profit. And, if insurers believe that those enrollment patterns will continue into 2015, then they may raise premiums higher to compensate for the loss.

However, because premiums are still allowed to vary substantially based on age, the financial consequences of lower enrollment among young adults are not as great as conventional wisdom might suggest.

To learn more on this topic, read the full report from the Kaiser Family Foundation.