Freund is best known for being the first to apply plethysmography (measurement of bloodflow) to the penis, thus permitting the first objective measurement of sexual arousal in males. Over his career, he refined the penile plethysmograph as part of a broad program of research on male sexual interest. Sex offenders could sometimes suppress arousal through concentration or surreptitiously causing themselves pain, similar to methods for producing false results on a polygraph (lie detector). However, Freund still felt that the plethysmograph remained the best measure of arousal (there was no evidence that subjects could consistently fake arousal, though they could sometimes suppress it.)