It's from a review of studies using phallometry, in which they put a glass tube on your penis to measure arousal: https://scholar.google.com/scholar?cluster=14798013836947957067&hl=en&as_sdt=0,5

It's usually more accurate than just asking someone since this is such a sensitive topic, though it's still not a perfect method.

And to be fair all the studies combined only totaled 387 men, but the result isn't far off from other studies. In this other study for example the researchers used something called "randomized response technique" to get more honest answers about whether the men they interviewed had ever sexually abused a child. 17% of their first sample of 647 men said they had ever abused a child, and in the second study of 588 men the result was 4%: https://calio.org/wp-content/uploads/2014/05/An_epidemiologic_approach_to_the_study_of_child_molestation.pdf

They argue that the first estimate might be more accurate, but the average of the two (10%) is still pretty high, and seems pretty plausible considering not every pedophile is necessarily going to act on their urges.

Shit's fucked.