The Homosexual Matrix the most novel account of the way in which behavioral expression communicates internal states since Charles Darwin's "The Expression of the Emotions in Man and Animals" (1872), but considered his most original contribution to be his explanation of how erotic feeling depends on "resistance", which might consist of any of several impediments to easy access to sexual satisfaction. Begelman called Tripp a skillful writer who was learned on the subject of homosexuality, but whose objectivity was nevertheless open to question. Karlen wrote that Tripp made numerous unjustified claims, such as that homosexual men tend to have a larger than average
The Expression of the Emotions in Man and Animals with engravings of the Darwin family's domestic pets by the zoological illustrator T. W. Wood as well as work by the artists Briton Rivière, Joseph Wolf and A.D. May. It also included portraits by the Swedish photographer Oscar Rejlander (1813–1875), anatomical diagrams by Sir Charles Bell (1774–1842) and Friedrich Henle (1809–1885), as well as illustrational quotations from the "Mécanisme de la physionomie humaine" (1862) by the French neurologist Duchenne de Boulogne (1806–1875). As a result of his domestic psychology experiments, Darwin reduced the number of commonly observed emotions from Duchenne's calculation of more than sixty facial expressions, to just six