For decades, runaway Medicare spending was the story of the federal budget.

Now, flat Medicare spending might be a bigger one.

Something strange has been happening in this giant federal program. Instead of growing and growing, as it always had before, spending per Medicare beneficiary has nearly leveled off over more than a decade.

The trend can be a little hard to see because, as baby boomers have aged, the number of people using Medicare has grown. But it has had enormous consequences for federal spending. Budget news often sounds apocalyptic, but the Medicare trend has been unexpectedly good for federal spending, saving taxpayers a huge amount relative to projections.

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The reason for the per-person slowdown is a bit of a mystery. Scholars have been arguing about it for years, but no one seems sure enough to confidently predict whether it is likely to stick around for much longer.