One of the big Republican ideas for reforming Medicare is to turn it into a program of vouchers that beneficiaries can use to purchase private insurance. You see this in Paul Ryan's plan, for instance. But that's not how these reforms save money. They save money by limiting the generosity of the vouchers. Because the dirty little secret is that turning Medicare into a voucher program would actually make it cost much, much more. How do we know? Well, putting aside the fact that Medicare currently costs much less than private insurance, we actually have a private voucher program in Medicare as we speak. Austin Frakt explains what it's taught us:

Though rarely described this way, the private Medicare Advantage plans are a (voluntary) voucher system. When covering a beneficiary, an Advantage plan receives a fixed monthly payment from Medicare that depends on the beneficiary's county of residence and health status. That fixed monthly payment is tantamount to a voucher. With it, beneficiaries can select from any Advantage plan operating in their county. They can also stick with traditional fee-for-service Medicare -- and about three in four beneficiaries do so.

But today, the market-based arm of the program costs more, not less, per beneficiary. Those fixed monthly payments to Advantage plans are, on average, 13 percent above fee-for-service Medicare costs. ...

What's going on? Why is the market-based Advantage voucher system not helping to control Medicare costs? The answer is that health care cost control is tough, technically and politically. Provider groups typically resist it. When it pertains to Medicare, beneficiaries resist it too. By adding another private-sector layer to the program -- health insurers -- the Advantage program invites a third source of political pressure. Rent-seeking by providers and insurers, as well as the power of the beneficiary constituency, align in their encouragement of higher Advantage payments. Congress, apparently, is willing to yield to that encouragement.