Freund was initially commissioned to use penile plethysmography to detect recruits attempting to evade military service by falsely claiming to be homosexual. (The Czechoslovak Army barred homosexuals from serving.) His larger research program, however, focused on detection and diagnosis of sex offenders, particularly preference pedophiles, with a view to more appropriate treatment guidelines. Freund was also involved in administering conversion therapy: "Freund made a large-scale attempt in Czechoslovakia in the 1950s to change the sexual orientation of homosexuals to a heterosexual orientation through the use of behavioral aversive therapy." His empirical data also showed some of the first evidence that sexual orientation conversion therapy was generally futile. He demonstrated that even homosexually oriented men who appeared to have given up sexual relations with other men and established heterosexual marriages were still aroused by images of men rather than women. Freund also challenged contemporary psychoanalytic theories of male homosexuality that suggested it was due to a fear or aversion to women. Freund concluded that homosexual men simply lacked erotic interest in females. Based on these studies, he advocated the decriminalization of homosexuality in Czechoslovakia (which took place in 1961) and the end of conversion therapy. These opinions also put him out of favor with the psychoanalytically dominated psychiatric establishment in Toronto, as he continued to argue that homosexuals needed understanding and acceptance rather than treatment. His efforts, along with those of German sexologist Rudolf Klimmer, helped decriminalize homosexuality in East Germany in 1968.