Parity has been the law since 2008. The data to measure it is finally here.

For tens of millions of Americans, finding an in-network therapist is harder than it should be. The search often takes weeks, with outdated provider directories and limited availability. The options that remain are familiar: pay out of pocket, go out of network, or delay care.

This is now how the system is supposed to work.

Since 2008, the Mental Health Parity and Addiction Equity Act (MHPAEA) has required that insurers cover mental health and substance use care on terms no more restrictive than physical health care. The law was clear in its intent – behavioral health should be treated like any other form of care – but uneven in its impact.

Americans remain more than 10 times more likely to go out of network for mental health care than for physical health care. For nearly two decades, parity enforcement has relied largely on anecdote, complaint, and litigation – not on standardized, comparable data. Regulators, employers, and even health plans themselves have lacked a clear way to see where parity is breaking down in practice.

That measurement gap has been the central challenge of parity.

Federal and state policymakers have been engaged on parity for some time. In September 2024, the federal government issued a new rule strengthening enforcement of MHPAEA, raising expectations for how plans evaluate and demonstrate compliance. At the same time, new transparency requirements have made more detailed data on pricing and networks available than ever before.

What has been missing is a way to turn that data into a clear, shared view of how parity is functioning across plans and geographies.

The Mental Health Parity Index (MHPI) is built to close that measurement gap. The MHPI was first piloted in Illinois — where 58 percent of residents are commercially insured and at least a quarter report symptoms of mental health conditions — before expanding nationally.

Unveiled on April 20, 2026 in Washington, D.C. by The Kennedy Forum in collaboration with Third Horizon, the American Medical Association, the American Psychological Foundation, and Ballmer Group, the MHPI turns a complex policy issue into something anyone can explore. At its core, MHPI examines two questions:
1. How does in-network behavioral health access compare to physical health access?
2. How do reimbursement rates compare between the two?

Users can visit parityindex.org, select a state or county, and see where meaningful differences emerge—bringing a long-standing policy issue into clearer focus.

The early findings are significant. Across the four largest commercial insurers — Aetna, BlueCross BlueShield, Cigna, and UnitedHealthcare — 43 states show potential disparities in access to in-network mental health and substance use care relative to physical health care. Seven in 10 counties show the same pattern. All 50 states show lower outpatient behavioral health payment rates than physical health rates within the same plans, with differences ranging from 16 to 59 percent depending on the network.

The relationship between reimbursement and access is well established. When payment rates are lower, fewer providers contract with carriers. When fewer providers are in network, patients have a harder time finding timely care. The MHPI makes this dynamic visible at the plan level for the first time, opening the door to a more informed conversation among insurers, employers, regulators, and consumers.

Parity has never been a question of whether the standard exists. It has been a question of whether the standard can be measured and enforced.

By making differences in access and reimbursement visible, the MHPI creates a shared foundation for action. Employers can better evaluate plan performance. Regulators can more easily identify potential compliance issues. Insurers and providers can engage with a clearer understanding of where gaps persist.

As Greg Williams, President of Third Horizon, noted at the launch, “the Index gives health plans, employers, policymakers, and providers a clearer view of where parity breaks down in practice.”

This is the kind of gap Third Horizon works to close. The MHPI represents an 18-month collaboration to take newly available transparency data and turn it into a tool that insurers, employers, providers, regulators, and patients can use. Third Horizon’s role has been the data-intensive analytic work — translating raw payer files into a clear picture of how parity is taking shape across plans and geographies.

In this month’s Impact Story, Third Horizon’s Erica Bauer sits down with Nathaniel Counts of The Kennedy Forum and health services researcher Andy Wilson of Third Horizon to talk about what the MHPI measures, why reimbursement matters, and what real progress will look like.

Watch the conversation.