Ask any budget expert in Washington to explain the ballooning deficit and debt, and Social Security and Medicare will be high on the list of causes. That’s wrong. The real driver, the elephant in the room, is means-tested social-welfare spending—Medicaid, food stamps, refundable tax credits, Supplemental Security Income, Temporary Assistance for Needy Families, federal housing subsidies and almost 100 other programs whose eligibility is limited to those below an income threshold. True, Social Security and Medicare are a drain on general revenue and will become big fiscal problems if not reformed. But they aren’t the major source of our current...