Dr. Robert C. Worstell, Dorothea Brande, Eark Nightingale, Claude M. Bristol, Napoleon Hill (2017). “Mindset StackingTM Inspirational Journal VolumeSS02”, p.144, Lulu.com
"Resolving social conflicts; selected papers on group dynamics" edited by Gertrude W. Lewin New York: Harper & Row, 1948, p. 133; as cited in "Schooling & Capitalism: A Sociological Reader" by Roger Dale, Madeleine MacDonald, Geoff Esland, (p. 111), 1976.