example, the gender gap is less pronounced in fraud cases than in drug trafficking and firearms. This disparity occurs in US federal courts, despite guidelines designed to avoid differential sentencing. The death penalty may also suffer from gender bias. According to Shatz and Shatz, "[t]he present study confirms what earlier studies have shown: that the death penalty is imposed on women relatively infrequently and that it is disproportionately imposed for the killing of women". There have been several reasons postulated for the gender criminal justice disparity in the United States. One of the most common is the expectation that women are predominantly care-givers. Other possible reasons include the "girlfriend theory" (whereby women are seen as tools of their boyfriends), the theory that female defendants are more likely to cooperate with authorities, and that women are often successful at turning their violent crime into victimhood by citing defenses such as postpartum depression or battered wife syndrome. However, none of these theories account for the total disparity, and sexism has also been suggested as an underlying cause. Gender discrimination also helps explain the differences between trial outcomes in which some female defendants are sentenced to death and other female defendants are sentenced to lesser punishments. Phillip Barron argues that female defendants are more likely to be sentenced to death for crimes that violate gender norms, such as killing children or killing strangers. Transgender people face widespread discrimination while incarcerated. They are generally housed according to their legal birth sex, rather than their gender identity. Studies have shown that transgender people are at an increased risk for harassment and sexual assault in this environment. They may also be denied access to medical procedures related to their reassignment. Some countries use stoning as a form of capital punishment. According to Amnesty International, the majority of those stoned are women and women are disproportionately affected by stoning because of sexism in the legal system. One study found that: [O]n average, women receive lighter sentences in comparison with men... roughly 30% of the gender differences in incarceration cannot be explained by the observed criminal characteristics of offense and offender. We also find evidence of considerable heterogeneity across judges in their treatment of female and male offenders. There is little evidence, however, that tastes for gender discrimination are driving the mean gender disparity or the variance in treatment between judges., A 2017 study by Knepper found that