"So what about the so-called trust fund? It’s often misunderstood. Think of it as an IOU or a promise to pay. When Social Security collected more money than it needed to provide benefits, as it did before 2010, the surplus wasn’t saved in a vault or invested. Instead, the government spent it on other expenses, and the Department of the Treasury wrote an IOU note to the Social Security Administration. Now that Social Security is cashing in those IOUs to cover the shortfall between income and expenses, it’s like borrowing money to pay off credit cards—the trust fund adds to the government’s overall debt.
Part of the confusion comes from thinking of Social Security and the Treasury as separate entities, with Social Security having assets on its balance sheet that the Treasury is on the hook for. However, taxpayers are on the hook for the government’s spending and borrowing. Since Social Security and the Treasury are both part of the government, we should think of them as one entity to fully understand Social Security’s financial impact on the American people."