The ethnicity of the composer Joseph Haydn was a controversial matter in Haydn scholarship during a period lasting from the late 19th to the mid 20th century. The principal contending ethnicities were Croat and German. Mainstream musical scholarship today adopts the second of these two hypotheses.
During the late 19th century, the Croatian ethnologist Franjo Kuha? gathered a great number of Croat folk tunes in fieldwork. Kuha? was struck by the resemblance of a number of these tunes to themes found in Haydn's works, and suggested that Haydn knew these tunes and incorporated them into his work. Other scholars disagreed, suggesting instead that the Haydn original themes had circulated among the people, evolving gradually into more folk-like forms. For details and examples, see Haydn and folk music.
Haydn never set foot in Croatia, but he almost certainly lived in the vicinity of Croatian speakers. This is because migration in previous centuries had resulted in a considerable number of Croats dwelling far to the north of Croatia in the Austro-Hungarian border region where Haydn was born and spent most of his life. This aspect of Kuha?'s claim is considered uncontroversial, though the relative fraction of the population that was Croatian-speaking is in dispute.
Kuha? went on to claim that the reason Haydn used so many Croatian folk tunes in his music is that he was himself a Croat; that is, a member of the Croat diaspora. As such, he would have been a native speaker of Croatian and a participant in Croatian folk culture. Kuha? also claimed that the name "Haydn" is of Croatian origin ("Hajdin"), and likewise for the name of Haydn's mother, Maria Koller.
Kuha? wrote in Croatian, which would have been a barrier to scholarly transmission at the time. However, his works were studied by the English-speaking musicologist Henry Hadow, who promulgated them further in his book Haydn: A Croatian Composer (1897) and in the second and third editions (1904-1910; 1927) of the prestigious Grove Dictionary.
In the 1930s, the German musicologist Ernst Fritz Schmid took up the issue of Haydn's origins, searching in parish records and elsewhere for evidence of Haydn's ancestry. He concluded on the basis of his research that Haydn's ethnic roots were not Croat, but German, and that the names "Haydn" and "Koller" are of German origin.
Schroeder (2009) describes Schmid's work as entirely convincing; "an enormously detailed examination of Haydn's genealogy" that "put all [alternative] theories to rest." He notes further, however, that the "timing of this type of study was unfortunate. Only a few years later similar genealogical studies affirming the German (and Aryan) roots of the Germanic musical giants had become a musicological preoccupation as a propaganda service to the National Socialist government ... Schmid's book pre-dates those sponsored by Propaganda Minister Joseph Goebbels and his chief ideologue Alfred Rosenberg, but it is a matter of regret that his proved to be a model for those which followed."
Despite its acquired associations with National Socialist musicology, Schmid's work has over the long term convinced not just Schroeder, but all mainstream Haydn scholarship.
Karl Geiringer endorses Schmid's views, both in his Haydn biography and in the fourth edition of the Grove Dictionary. In the 1982 revision of his biography, Geiringer wrote
Schmid's views were also endorsed by the French scholar, Michel Brenet, and by Rosemary Hughes in their Haydn biographies. H. C. Robbins Landon devoted the opening pages of his massive work Haydn: Chronicle and Works to a long summary and warm endorsement of Schmid's research.
In the current version of the Grove Dictionary, the Haydn biography (by James Webster) does not even mention the old controversy, other than to cite Schmid's work in the bibliography. Neither Kuha? nor Hadow is cited.
Curiously, Haydn himself is recorded as having made a somewhat disparaging remark about Croats. His words were remembered by the composer and pianist Friedrich Kalkbrenner, who was Haydn's student in Vienna around the year 1800; he wrote them down in his memoirs, published 1824. In the memoirs, Kalkbrenner refers to himself in the third person.