Macro-Siouan languages
In the 19th century, Robert Latham suggested that the Siouan languages are related to the Caddoan and Iroquoian languages. In 1931, Louis Allen presented the first list of systematic correspondences between a set of 25 lexical items in Siouan and Iroquoian. In the 1960s and 1970s, Wallace Chafe further explored the link between Siouan and Caddoan languages. In the 1990s, Marianne Mithun compared the morphology and syntax of all the three families. At present, the Macro-Siouan hypothesis based on relations among Siouan, Caddoan, and Iroquoian is not universally accepted as proven.
Languages of the United States
Dakota is a Siouan language with 18,000 speakers in the US alone (22,000 including speakers in Canada), not counting 6,000 speakers of the closely related Lakota. Most speakers live in the states of North Dakota and South Dakota. Other Siouan languages include the closely related Winnebago, and the more distant Crow, among others.