Table 2 presents a list of selected topics and the prevalence of contracts and executive agreements in each area. A complete list of agreements by theme can be found in the online annex. The only subject on which treaties are more frequent than executive agreements is extradition, where 94 per cent of agreements are concluded in the form of contracts. One likely explanation for this phenomenon is uncertainty about the constitutionality of using executive agreements to hand over individuals to foreign nations. The uncertainty stems from Valentine v. United States, footnote 94 where the Supreme Court ruled that extraditions should be authorized “by the law of Congress or by the terms of a treaty.” But Valentine did not check whether extraditions under agreements between Congress and the executive branch were constitutional.