Why Would the Addition of Young Healthy People Raise the Cost of Insurance?

That's what Washington Post readers are asking after reading an article warning of large price increases in 2017. The piece reported the output from a simulation model developed by Stephen Parente, a University of Minnesota health economist who advised Sen. John McCain's 2008 presidential campaign.

According to the article, Parente's model predicts a large increase in the price of bronze and catastrophic plans in the exchanges in 2017. It gives the reason for this increase:

"Parente estimate...