Center on Budget and Policy Priorities article arguing that the percentage of income that typical middle-class families pay in federal taxes is often overstated. http://www.cbpp.org/1-21-99taxburden.htm
Study found that most states take a far greater share of income in taxes from middle- and low-income families than from the wealthy. http://www.ctj.org/html/whopay.htm
Essay discussing the concepts of progressivity in taxation, the role of taxation in regulating the economy, and explaining why sales taxes are considered to be regressive. http://www.psnw.com/~bashford/taxation.html