You don’t have to be a perve to wish Kevin Sandler could flash a little more skin in his thought-provoking but doggedly academic treatment of the movie ratings system, “The Naked Truth.” The Arizona professor is on to a genuinely fascinating topic that’s as old as the Hollywood Hills. It has always seemed deliciously loony that the film industry attempts to keep score such that, as one vexed producer put it, “four fucks, two goddamns and one motherfucker is an R.” Getting to the four-letter fun in this case doesn’t require a parent or guardian, but rather a high tolerance for footnotes and second-hand sources.The author at least is not laboring under false pretenses. “‘The Naked Truth’ merely scratches the surface,” he concedes, due to the imposing fact that little of the ratings process has ever been revealed through official channels. The number of authoritative accounts of the secret process by which movies have been evaluated and rated over the past 40 years can be counted on one hand. Given that limited source pool, or presumably his own ability to go on sabbatical and follow Kirby Dick with a notebook, Sandler dutifully lays out a chronology dating back to before the Hays Code. Along the way, he tosses off some astounding facts. For instance, after Columbia’s “Emmanuelle” in 1974 and United Artists’ “Inserts” in 1976, MPAA signatories put out just two X-rated pics before 1990′s “Henry & June,” and both were Pasolini re-releases. After “Henry,” only indies, specialty units or foreign distribs have attempted NC-17s, generally with limited return. That de facto shut-down of the X rating, and the subsequent NC-17, are detailed in convincing fashion, and there are flashes of passion as Sandler assails the “cosmetic” changes to a broken ratings system. Too often, though, the writing is wonky and labored, with repeated references to full source citations and sources “remarking” and “observing” as if in a photo caption come to life.