Medicare Advantage now enrolls more than half of all Medicare beneficiaries. In 2007, that figure was roughly 19 percent. This shift from traditional fee-for-service Medicare to privately administered plans represents one of the most consequential changes in American healthcare financing in a generation—and it happened largely without a single landmark law driving it.

The growth engine is built into Medicare's payment architecture itself. Private plans receive per-beneficiary payments from the federal government, calibrated through a system of benchmarks and risk scores. Understanding how that payment system works—and where its incentives point—is essential for evaluating whether this expansion is delivering value or quietly inflating costs.

Three interconnected policy mechanisms deserve close examination: how Medicare sets payment rates for private plans, how the enrollment patterns of those plans affect spending comparisons, and how the risk adjustment system designed to ensure fairness has itself become a source of concern. Each reveals something important about the gap between policy design and real-world outcomes.

Medicare Advantage plans don't bill the government for each service they deliver. Instead, the Centers for Medicare and Medicaid Services pays each plan a fixed monthly amount per enrollee, determined through a bidding process measured against a local benchmark. That benchmark is derived from historical per-capita spending in traditional Medicare for a given geographic area—but it isn't a simple mirror of those costs.

The Affordable Care Act restructured these benchmarks into a tiered system, linking payment levels to each county's traditional Medicare spending relative to the national average. Counties with lower traditional Medicare spending receive benchmarks set above local fee-for-service costs, while high-spending counties receive benchmarks closer to or below those costs. The intent was to encourage plan participation in efficient areas while constraining overpayment in expensive ones.

Plans submit bids reflecting what they estimate it will cost to cover the standard Medicare benefit package. If a plan bids below the benchmark, it keeps a share of the difference as a "rebate," which it must return to enrollees in the form of supplemental benefits—dental coverage, vision care, reduced cost-sharing, or fitness programs. These extras are a major driver of the program's appeal to beneficiaries.

Here's the structural tension: the Government Accountability Office and the Medicare Payment Advisory Commission have repeatedly found that total federal payments to Medicare Advantage plans exceed what those same beneficiaries would have cost in traditional Medicare. Estimates suggest the overpayment ranges from 6 to 25 percent depending on methodology and year. The benchmark system, in other words, was designed to introduce competition and efficiency. In practice, it has often functioned as a subsidy—one that funds attractive supplemental benefits while costing taxpayers more per person than the public program it was meant to improve upon.

Takeaway

A payment system designed to reward efficiency can quietly become a subsidy when the benchmarks it relies on are set above the costs they're supposed to reflect.

Medicare Advantage plans have strong incentives to attract enrollees who are healthier than average. Healthier members use fewer services, which means a plan collecting a fixed per-member payment keeps more of that payment as margin. This dynamic—known as favorable selection—has been a persistent concern since the earliest days of private Medicare contracting in the 1980s.

The evidence is substantial. Research consistently shows that beneficiaries who switch into Medicare Advantage tend to have lower prior-year healthcare spending than those who remain in traditional Medicare. Conversely, beneficiaries who disenroll from Medicare Advantage and return to fee-for-service tend to be sicker and more costly. This creates a pattern where private plans systematically capture lower-cost individuals while the traditional program absorbs a disproportionate share of complex, expensive patients.

Plans don't need to explicitly screen out sick enrollees—benefit design does much of the work. Generous gym memberships and wellness programs appeal to relatively healthy seniors. Narrow provider networks may deter beneficiaries with established relationships with specialists for chronic conditions. Prior authorization requirements can discourage enrollment by people who anticipate needing intensive services. None of these features violate any rule, yet their cumulative effect shapes the risk pool.

This selection pattern undermines the fundamental spending comparison between Medicare Advantage and traditional Medicare. If private plans enroll systematically healthier people, then the apparent cost savings they generate aren't necessarily evidence of greater efficiency—they may simply reflect a healthier membership base. Adjusting for this selection bias is technically difficult and politically contentious, which is precisely why it persists as an unresolved problem in Medicare policy.

Takeaway

When a system compares costs across two pools without fully accounting for who is in each pool, apparent efficiency gains can be an illusion created by the composition of the groups themselves.

To counteract favorable selection, Medicare uses a risk adjustment system called the CMS Hierarchical Condition Categories model. The idea is straightforward: plans that enroll sicker beneficiaries receive higher payments. Each enrollee's risk score is calculated based on their documented diagnoses, demographic characteristics, and eligibility status. A beneficiary with diabetes, heart failure, and chronic kidney disease generates a higher payment than one with no significant conditions.

The problem is that risk adjustment has created a powerful financial incentive to document more diagnoses—not to fabricate them, but to ensure that every existing condition appears in a beneficiary's medical record. Medicare Advantage plans invest heavily in chart reviews, health risk assessments, and in-home visits specifically designed to identify and code conditions that might otherwise go unrecorded. In traditional Medicare, providers have no comparable incentive because payment is based on services rendered, not diagnoses carried.

The result is a measurable divergence in coding intensity. Research from the Government Accountability Office and academic health economists has found that Medicare Advantage risk scores grow faster than those of comparable traditional Medicare beneficiaries, even after controlling for health status changes. This coding intensity gap translates directly into higher payments. Estimates from the Medicare Payment Advisory Commission suggest that coding intensity adds roughly 4 to 8 percent to Medicare Advantage payments beyond what clinical reality would justify.

CMS applies a coding intensity adjustment—a blunt across-the-board reduction to Medicare Advantage risk scores—to partially offset this effect. But the adjustment has consistently lagged behind the actual magnitude of the problem. Plans continue to invest in coding infrastructure because the return on that investment remains substantial. The risk adjustment system, designed to make payments fairer, has instead become a major channel through which additional federal dollars flow to private plans without corresponding increases in the actual health needs of their enrollees.

Takeaway

A system that pays based on documented illness will inevitably produce more documented illness—not because people are sicker, but because documentation itself becomes economically valuable.

Medicare Advantage's growth isn't a market verdict on the superiority of private insurance. It's the predictable result of a payment architecture that combines generous benchmarks, favorable enrollment patterns, and a risk adjustment system that rewards aggressive coding. Each mechanism is individually defensible in theory. Together, they create a system that consistently transfers more federal dollars to private plans than equivalent coverage would cost in traditional Medicare.

None of this means Medicare Advantage delivers no value. Many beneficiaries genuinely prefer their plans' supplemental benefits and cost-sharing protections. But the policy question isn't whether enrollees are satisfied—it's whether taxpayers are paying a fair price for the coverage those plans provide.

Reforming these mechanics requires confronting the political reality that any change to benchmarks, risk adjustment, or enrollment rules threatens benefits that millions of seniors now rely on. The longer these structural subsidies persist, the harder they become to unwind.