The book has seven chapters describing the prevalence, nature and theories in the 19th century about sexual inversion in men and women.

"Homosexual practices in private, between two consenting parties, are absolutely unpunished [...]" in France, Belgium, the Netherlands and other European countries.

[3]: 5 In England, anal penetration of men and women as well as mutual masturbation for man was still sentenced even when it occurred privately, with consent and no involvement of minors.

[3]: 107  Their correspondence happened via letters and after Symonds' death, Ellis was not able to find a publisher for his book in England because they all feared prosecution.

[1]: 8  An observation by Dr. Holder describes these men as dressing and acting like women from childhood on, followed by sexual practices after puberty.

Also, some men of great intellect, as the author describes it, like the Roman Emperor Caesar or the French Humanist Muretus are known to have had some homosexual tendencies.

Among them is Karl Friedrich Otto Westphal a German psychiatrist from Berlin who proposed that sexual inversion is something innate and not acquired and is not necessarily a sign of insanity.

[1]: 25  Attempts to classify forms of sexual inversion are also discussed for example one proposal by Richard Krafft-Ebbing a psychiatrist and author of Psychopathia Sexualis.

[1]: 88  Furthermore, the author suspects that the prevalence of homosexuality among women is increasing which he explains by emancipation and equal rights movements.

[1]: 138  Suggested causes include the co-education of boys and girls, the seduction by an older person and disappointment in normal love.

[1]: 140 In the conclusion Ellis first examines the treatment of Dr. von Schrenck-Notzing who was a German physician, psychiatrist and notable psychical researcher in the field of paranormal events.

"[1]: 143  It seems to Ellis that it is not possible to "cure" the sexual invert but that there should be an effort made to help them being healthy, self-restrained and self-respecting to be functional members of society.

[1]: 150–54 The book contains the following appendices: The studies in the psychology of sex vol.1 is an example of a more diverse view of sexuality from the Victorian era.

But given the strict policing of knowledge and even the language used to discuss such intimate behaviours, we must ask – what did that tolerance really mean?The book was not published in England because it was noted that homosexual practices increase in Europe and as a consequence.

[6] Canadian-born British feminist, socialist, sex radical, and birth control campaigner Stella Browne, in her review of Volume II, says "England has already impoverished herself intellectually and covered herself with ridicule by the persecution of the original edition of this psychological classic.

[7] Although the conclusions due to a lack of data they already lead to an understanding of the fact that it does not make sense to try to change the instinct of sexual inverted people.

"[8] Reading is regarded as necessary for teachers, people working with criminals and parents to prevent sexual inversion from occurring.