Smith, Jonathan C. (2009). Pseudoscience and Extraordinary Claims of the Paranormal: A Critical Thinker's Toolkit. Wiley-Blackwell. (ISBN978-1405181228). "Today, researchers discount the first decade of Rhine's work with Zener cards. Stimulus leakage or cheating could account for all his findings. Slight indentations on the backs of cards revealed the symbols embossed on card faces. Subjects could see and hear the experimenter, and note subtle but revealing facial expressions or changes in breathing."
