Discussion among tax experts about replacing the federal income tax with either a national sales tax or a national value-added tax emphasizes some of the political and administrative obstacles of shifting to a tax system based on consumption rather than income, according to Steven Sheffrin, professor of economics and director of the Center for State and Local Taxation at UC Davis. Concerns about equity in the impact of the taxes, the likelihood of higher-than-expected rates, questions about the enforceability of, and compliance with, tax rates that Americans are not used to paying, and the very complexity of administering the resulting tax system, are some of the important issues embedded in a possible tax shift. "In general, the burden of consumption taxes would fall more on lower-and-middle-income households," says Sheffrin. "In addition, introducing taxation on consumption would impose a one-time additional burden on holders of accumulated assets."