NBC, CBS, Fox and CW did not comment on the study, which did not include any ABC shows.
Under political and social pressure in the mid-1990s, the voluntary system was established by the TV industry to be used with the so-called V-chip that can block shows electronically.
Networks find it financially vital to avoid applying TV-MA ratings, Winters said, which scare off advertisers.
To assess how the ratings are used, the PTC said it analyzed the seven shows each on cable and broadcast TV that had the highest levels of violence. Each show's first four episodes of the 2012-13 season were analyzed.
TV-14 warns that a program may include intense violence, sex or language not suitable for children under 14, while TV-MA is intended for shows that might have indecent language, graphic violence or explicit sexuality, according to the TV Parental Guidelines webpage.
The PTC study defined graphic as "especially vivid, brutal and realistic acts of violence" that are explicitly depicted. Among the network examples cited:
— A bar fight scene on NBC's "Revolution" in which a character wields a sword and a dagger to slash open a man's chest, cut another's neck and stab a third in the chest. The blood-spattered character pulls his sword from the last victim's body.
— CW's "Supernatural," in which a trail of blood leads to the bodies of two priests impaled on a cemetery's wrought-iron fence. Their eyes have been gouged out and blood drips down their faces.
— A woman is tortured in captivity, with an implanted camera sending images of her agony online in an episode of CBS' "Criminal Minds." An FBI agent watches as a hammer is driven into the victim's head.
Depictions of shootings, stabbings and dismemberment were found on cable shows including AMC's "The Walking Dead" and "Breaking Bad" and FX's "Sons of Anarchy." Five of the seven cable shows had TV-MA ratings, with "Walking Dead" eventually switching from TV-14 to MA.