In 1998, Joseph Greenberg extended his work in mass comparison, a methodology he first proposed in the 1950s to categorize the languages of Africa, to suggest a Eurasiatic language. In 2000, he expanded his argument for Eurasiatic into a full-length book, "Indo-European and Its Closest Relatives: The Eurasiatic Language Family", in which he outlines both phonetic and grammatical evidence that he feels demonstrate the validity of language family. The heart of his argument is 72 morphological features that he judges as common across the various language families he examines. Of the many variant proposals, Greenberg's has attracted the most academic attention.