Havelock Ellis (2013). “Psychology of Sex: The Biology of Sex—The Sexual Impulse in Youth—Sexual Deviation—The Erotic Symbolisms—Homosexuality—Marriage—The Art of Love”, p.7, Butterworth-Heinemann
"Kenneth Ewart Boulding: 1910-1993. An Appreciation" by Robert A. Solo, Journal of Economic Issues Volume 28, No. 4, pp. 1187-1200, www.jstor.org. December 1994.