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, this Macro-Siouan hypothesis is not considered proved, and the similarities between the three families may instead be due to their protolanguages having been part of a sprachbund.
Mandan
The Mandan language or "Nų́ų́ʔetaa íroo" belongs to the Siouan language family. It was initially thought to be closely related to the languages of the Hidatsa and the Crow. However, since the Mandan language has been in contact with Hidatsa and Crow for many years, the exact relationship between Mandan and other Siouan languages (including Hidatsa and Crow) has been obscured. For this reason, linguists classify Mandan most often as a separate branch of the Siouan family.