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 shows how human emotions link mental states with bodily movement, and are genetically determined, deriving from purposeful animal actions. He comments on the implications of the book: a single origin for the entire human species, with universal human expressions; and he stresses the social value of expression, citing the emotional communication between mother and child. This was one of the first books to be illustrated with photographs – with seven heliotype plates – and the publisher John Murray warned that this "would poke a [terrible] hole in the profits". The published book assembled illustrations rather like a Victorian family album,