I'm imagining a very awkward medical study trying to verify this claim. How could one actually generalize and quantify such an intrinsically personal and almost by definition individual activity?

As a circumcised non-Jewish white American man, I can't know how difficult it would have been for me to masturbate if I was not circumcised, but I can say that being circumcised didn't ever hinder me from peeling the carrot, choking the chicken, spanking the monkey, wrestling the one-eyed wonder worm, or any of the other hilarious euphamisms for masturbation.

I do know from a youth filled with team sports that white Americans born in the mid-60s were pretty much ALL circumcised.