The prevailing medical consensus is that drug addiction and alcoholism are disabilities. Before 1996, SSI and SSDI, the nation’s major disability benefits programs, recognized that consensus and provided benefits to people struggling with addiction. Then, the “DAA materiality” provision of Congress’s 1996 welfare reform legislation revoked eligibility not only from people struggling with addiction, but also from people with addiction and another severe disability whose addiction contributes to the severity of the other disability. For this latter group of “dual-diagnosis” claimants, it is often impossible to determine which of a claimant’s impairments would remain absent substance abuse. In such cases, the evidence is in equipoise, and whichever party bears the burden of proof of DAA materiality will lose. Despite its importance to many disability benefits claimants, the issue of who bears the burden of proof remains unresolved, with the Social Security Administration placing the burden on the government and a split among the federal appeals courts that have taken up the issue. This Note argues that the burden of proof of DAA materiality should fall on the government. It shows that the DAA materiality provision creates an exception to the definition of disability in the Social Security Act that functions like an affirmative defense for the government to deny benefits to otherwise eligible claimants. It then contrasts the many obstacles facing dual-diagnosis claimants with the government’s superior resources and expertise to offer proof on the complex DAA materiality issue.