Paraphilic infantilism, also known as autonepiophilia, psychosexual infantilism and adult baby syndrome, is a sexual fetish that involves role-playing a regression to an infant-like state. Behaviors may include drinking from a bottle or wearing diapers (diaper fetishism). Individuals may engage in gentle and nurturing experiences (an adult who engages only in infantile play is known as an adult baby) or be attracted to masochistic, coercive, punishing or humiliating experiences. Diaper fetishism involves "diaper lovers" wearing diapers for sexual or erotic reasons but may not involve infant-like behavior. Individuals who experience both of these things are referred to as adult baby/diaper lovers (AB/DL, also spelled ABDL). When wearing diapers, infantilists may urinate and/or defecate in them.

There is no recognized etiology for infantilism and there is little research done on the subject. It has been linked to masochism and a variety of other paraphilias. Although it is commonly confused with pedophilia, the two conditions are distinct and infantilists do not seek children as sexual partners. A variety of causes have been proposed, including altered lovemaps, imprinting gone wrong and errors in erotic targets, though there is no consensus. A variety of organizations exist to discuss infantilism or meet with other practitioners throughout the world.