Patricia Hill Collins, an African-American writer, theorizes that this is derived from a stereotype that those of black descent are promiscuous, and their buttocks are objectified by porn as a result. Studies have found that masturbation is frequent in humans of both sexes and all ages, although there is variation. It is also followed by dedicated industry publications and trade groups as well as the mainstream press, private organizations , government agencies, and political organizations. In humans, females generally have more round and voluptuous buttocks, caused by estrogen that encourages the body to store fat in the buttocks, hips, and thighs. The warm sensation felt when urine trickles on the body seems to give very relaxing and pleasurable feelings to the person. Participants who exert sexual control over their partners are known as dominants or tops while participants who take the passive, hentai videos receiving, or obedient role are known as submissives or bottoms. Works in this genre may be considered adult for any number of reasons. The term is typically used by the cinematographer within the narrative framework of a pornographic film, and since the 1970s has become a leitmotif of the hardcore genre. A related term used in the genre is the term cougar, which implies an older woman as predator. John Stagliano is considered to have started the gonzo pornography genre with his Buttman series of films. Bear features a combination of middle-aged, hairy or overweight males. Though there have been numerous gang bang pornographic films since the 1980s, they usually involved no more than half a dozen to a dozen men. Sexual orientation Pornography features an increasing gay sex video variety of combinations of actors by their sexual orientation such as lesbian, gay, bisexual, and transsexuals in addition to traditional heterosexual roles. The existence of such content in commercially nude milfs available pornography is widely considered an urban legend. Examples of mentally oriented practices are education games, during which the dominant requires certain forms of behavior from the submissive.