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. Other objects such as ball point pens and thermometers are sometimes used, although this practice can lead to injury and/or infection. Older names for a pornographic film include stag film, and blue movie. Dwarfs or midget pornography features performers who are dwarfs/midgets. The most common type of erotic spanking is administered on the bare buttocks, but can also be combined with bondage, in order to heighten sexual arousal and feelings of helplessness. Snowballing or snowdropping is the human sexual practice in which one person takes someone else semen into his or her mouth and then passes it to the mouth of the other, usually through kissing. A homosexual threesome would involve either three men or three women. This 3d hentai industry employs thousands of performers along with support and production staff. Sigmund Freud considered foot binding as a form of fetishism. Transgender pornography features performers who cross-dress for sexual gratification, either for themselves or for their partners. Pro-am pornography is sub-genre of amateur pornography that refers to professionally made porn that features amateur actors/actresses, often appearing for the first time in a pornographic film. Concerted deployed sexual rejection exercised on the partner can be an aspect of Dominance and Submission as well. Hermaphrodite, also known as herm, refers to pornography, typically hentai, that features persons with the genitalia of both sexes. The porn industry and viewing public momporn were shocked by the then-taboo spectacle of a white woman having sex with a black man. The existence of such gay boy porn content in commercially available pornography is widely considered an urban legend. The term pearl necklace is used by some because the drops of semen deposited by the man resemble a necklace of translucent white pearls.