Diana Baumrind: "The scientific case against the use of normative physical punishment is a leaky dike."
George Ryley Scott: "There has probably never been an evil of any description for which it was not possible to dig up some argument in its favour, some excuse for its continued existence. The justification for the existence or extension of a practice lies not in the fact that it possesses certain virtues, but that those virtues outweigh its drawbacks, or that it serves society in some essential form for which there is no alternative method available. By no species of argument can anything of this nature be claimed for corporal punishment. Its evils, its drawbacks, and its disadvantages, as we have seen in the process of our inquiry, outweigh hugely and in every possible way, its few virtues— virtues which are based upon the most dubious foundations."
Diana Baumrind's paper "Does Causally Relevant Research Support A Blanket Injunction Against Disciplinary Spanking By Parents?" and Data Tables handout can be viewed in PDF format, using Acrobat Reader. Go to http://ihd.berkeley.edu/BaumrindPaper.pdf.