One of the best indicators of men's involuntary sexual arousal in response to images is phallometry which consists in measuring penile tumescence. Multiple studies have been done to assess the male arousal profile in response to erotic stimuli featuring females of various ages.
One of the first such studies by Freund and Costell (1970) involved 48 healthy young men of average age 20 enrolled in compulsory military service with no prior history of any mental or physical problems. These men underwent phallometry during the presentation of pictures of naked males and females in three age groups: children (4-10 yrs), adolescents (12-16 yrs) and adults (17-36 yrs).
The strongest penile response was found for adult and adolescent females without significant difference between the two groups, with about 83-88% of males experiencing sexual arousal. Furthermore, 52-58% of healthy men experienced a positive penile response indicating some level of arousal to female children. The control response to males of all ages was low.
This result has since been replicated many times over the following decades.
Hall (1995) examined pedophilic stimuli among N = 80 men drawn from a community sample of volunteers recruited by newspaper advertisement. He found that 20% of men self-reported some level of sexual interest in pre-pubescent female children and 26.25% exhibited strong (equal or greater than their arousal to adult female erotic stimuli) objective penile arousal to sexual stimuli that was clearly stated as depicting pre-pubescent female children.
All of the men in the study had reported prior sexual contact with adult women, and the authors of the study noted that sexual arousal to children was not necessarily predictive of actually going on to commit child sexual abuse, as prior research had demonstrated that a large amount of child molesters do not actually exhibit a "deviant" sexual arousal pattern whatsoever (in terms of being exclusively or equally attracted to children vs adult women), when monitored.
Similarly, Becker-Blease et al. (2006) found in a sample of N = 531 undergraduate men that approximately 18% report having fantasized in the last 12 months about perpetrating child sexual abuse (e.g. "how often have you had fantasies about sex with a child?") and 8% had masturbated to these fantasies. Only a minority of men in the sample seemed to have any intentions of acting on these fantasies, however, 4% did indicate that they would possibly engage in sexual activity with a child if there was no risk of exposure.
Filip Schuster (2014) conducted a meta-analysis revealing that 22% of normal men show greater or equal sexual arousal to child stimuli (individuals up to 13 years old) than to adult stimuli. About 3% of men have a preference for pedophilia (mostly sexually aroused by prepubescents) and about 16% for hebephilia (mostly sexually aroused by pubescents).
The scientific consensus is that arousal by child pornography is not necessarily indicative of the paraphilia to be attracted to females far under the legal age of consent, and indeed, such arousal is normal.
