Unpublished research by Terrence Kaufman in the 1980s supported grouping Cuicatec and Mixtec together.

The thousand-year presence of Oto-Manguean-speaking groups in this region makes it probable that they were active in this domestication process, which favored the inhabitants of the Altiplano's transition to a sedentary lifestyle and thus influenced the development of Mesoamerican civilization.

[4] Campbell and Kaufman have proposed that the Oto-Manguean languages began to diverge about 1500 BCE.

[5] The groups of consonants and the diphthongs formed from this limited repertory would have been the origin of the phonemes in the daughter proto-languages of the various subgroups of Proto-Oto-Manguean.

He revised the probable phonological inventory and described some of his proposals, based on comparisons of the cognates in the Mixtecan languages.