If any type of study merits our attention, it is one shown on a little known website, done by a group of anyonymous "international researchers", and offering a flash animation in lieu of an explanation of their methodology.
Now, if you'll excuse me, I'm off to write a scientific paper on how White males living in Los Angeles with degrees in chemistry and +1,000 posts on a porn website have the most length, girth, and machine-like endurance among all types of potential sex partner. I'll let you know when Scientific American or JAMA publishes it.