Jane Gravelle (Congressional Research Service) & Jennifer Gravelle (Government Accountability Office) have published Taxing Poor Families: The Evolution of Treatment Under the Federal Income Tax, 7 Conn. Pub. Int. L.J. 191 (2008). Here is part of the Introduction:

Dramatic changes in the stance of the federal income tax towards low income individuals occurred during the history of the income tax. This article will outline the major shifts in the taxation of the poor, from the initial tax system, which included a large flat exemption and focused largely on higher income families, through a period of eroding exemptions and heavier taxation of the poor, to our current system with personal exemptions, deductions, and a negative tax via the Earned Income Credit (EIC).