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 language
Mandan was initially thought to be closely related to Hidatsa and Crow. However, since Mandan has had language contact with Hidatsa and Crow for many years, the exact relationship between Mandan and other Siouan languages (including Hidatsa and Crow) has been obscured and is currently undetermined. Thus, Mandan is most often considered to be a separate branch of the Siouan family.