Kent State Study
Sexual Arousal and Arousability to Pedophilic Stimuli in a Community Sample of Normal Men, aka the Kent State Study or Hall Study, written by Gordon Hall, Richard Hirschman and Lori Oliver of Kent State University is a phallometric study published in the Journal of Behavior Therapy (26, 681-694, 1995).
The study suggested that 33% and 26% of adult males (who responded to a newspaper advert) experienced equal or higher sexual arousal on the viewing of prepubescent sexual stimuli than adult stimuli, despite the elimination of men who admitted to sexual relations with children.
Caution
One thing that the study does not demonstrate is that 26-33% of the volunteers (let alone "normal men" in the general population) are pedophiles. In fact, the number of pedophiles in Hall's sample is probably much smaller due to factors such as the absence of adolescent stimuli (meaning that ephebophiles may be more aroused by prepubescent stimuli and also an over-representation of unmarried men and by implication, male homosexuals who may conceivably prefer small, androgynous girls to fully developed women.