children; the marketing of products with sexual connotations to children, including clothing; the lack of parental oversight and discipline; access to adult culture via the internet; and the lack of comprehensive school sex education programs. For girls and young women in particular, studies have found that sexualization has a negative impact on their "self-image and healthy development". Child sexual abuse Child sexual abuse is defined as an adult or older adolescent having a sexual relationship with a child. Effects of child sexual abuse include clinical depression, post-traumatic stress disorder, anxiety, propensity to further victimization in adulthood, and physical injury to the child, among other problems. Child sexual abuse by a family member is a form of incest, and can result in more serious and long-term psychological trauma, especially in the case of parental incest. Children who have been the victim of child sexual abuse sometimes display overly sexualized behavior, which may be defined as expressed behavior that is non-normative for the culture. Typical symptomatic behaviors may include excessive or public masturbation and coercing, manipulating or tricking other children into non-consensual or unwanted sexual activities, also referred to as "child-on-child sexual abuse". Sexualized behavior is thought to constitute the best indication that a child has been sexually abused. Children who exhibit sexualized behavior may also have other behavioral problems. Other symptoms of child sexual abuse may include manifestations of post-traumatic stress in younger children; fear, aggression, and nightmares in young school-age children; and depression in older children. Among siblings In 1980, a survey of 796 undergraduates, 15 percent of females and 10 percent of males reported some form of sexual experience involving a sibling; most of these fell short of actual intercourse. Approximately one quarter of these experiences were described as abusive or exploitative. A 1989 paper reported the results of a questionnaire with responses from 526 undergraduate college students in which 17 percent of the respondents stated that they had preadolescent sexual experiences with a sibling. Methodological issues Empirical knowledge about child sexual behavior is not usually gathered by direct interviews of children, partly due to ethical consideration. Information about child sexual behavior is gathered by the following methods: Observing children being treated for problematic behavior, such as use of force in sex play, often using anatomically correct dolls; Recollections by adults; Observation by caregivers. Most published sexual research material emanates from the Western World, and a great deal of