Many acknowledge the scarcity of research on female paraphilias. The majority of paraphilia studies are conducted on people who have been convicted of sex crimes. Since the number of male convicted sex offenders far exceeds the number of female convicted sex offenders, research on paraphilic behavior in women is consequently lacking. Some researchers argue that an underrepresentation exists concerning pedophilia in females. Due to the low number of women in studies on pedophilia, most studies are based from "exclusively male samples". This likely underrepresentation may also be attributable to a "societal tendency to dismiss the negative impact of sexual relationships between young boys and adult women". Michele Elliott has done extensive research on child sexual abuse committed by females, publishing the book "Female Sexual Abuse of Children: The Last Taboo" in an attempt to challenge the gender-biased discourse surrounding sex crimes. John Hunsley states that physiological limitations in the study of female sexuality must also be acknowledged when considering research on paraphilias. He states that while a man's sexual arousal can be directly measured from his erection (see penile plethysmograph), a woman's sexual arousal cannot be measured as clearly (see vaginal photoplethysmograph), and therefore research concerning female sexuality is rarely as conclusive as research on men. In the United States, since 1990 a significant number of states have passed sexually violent predator laws. Following a series of landmark cases in the Supreme Court of the United States, persons diagnosed with paraphilias, particularly pedophilia ("Kansas v. Hendricks", 1997) and exhibitionism ("Kansas v. Crane", 2002), with a history of anti-social behavior and related criminal history, can be held indefinitely in civil confinement under various state legislation generically known as sexually violent predator laws and the federal Adam Walsh Act ("United States v. Comstock", 2010).