The following map was issued by Encyclopedia Britannica, Edition 1990 (only a year before the civil wars broke in ex-Yugoslavia), Macropedia Volume 29, page 1090.
Ethnic composition of Yugoslavia
This map appears to be identical to the one published the same year in National Geographic. You can compare the two maps here. Even more interesting is the comparison between the two different editions of Encyclopedia Britannica which are only three years apart. Take a look at here presented ethnic map from edition 1986 (and repeated in edition 1987). What is the difference?
Only one year before the civil wars are to start - Muslims suddenly appear, for the first time, as a separate ethnic group! They are not "Bosniaks" yet, though. Evidently the authors anticipated a war where the Muslim Serbs will demand to be treated as a separate ethnic group. This is first such precedent that religion, and religion only, suffice as determining factor of ethnicity.
It was Communist Dictator Tito, the absolute ruler of Yugoslavia for 35 years and a man who liked being called "father of the Nations", who invented yet another nation the Bosnian Muslims. Before that the Croatian Communist invented "Macedonians" and Montenegrins (the two groups are actually ethnic Serbs that live in geographic regions of Macedonia and Monternegro). It was all part of the governing Communist formula: "Weak Serbia = strong Yugoslavia". In case of the Bosnian Muslims though, Tito made a dangerous international precedent. Bosnian Muslims speak the same language and are of identical ethnicity as Serbs. The only difference is - religion. Can religion, and religion only, be a reason for forming a separate nation? What would then happen to America for example? How many nations, how many countries should America split to?
Check the above claims (about language) by comparing this map with the map entitled "Principal Languages and Religions" that appear in book "Yugoslavia, a country study" published in 1982 by Headquarters, Department of the Army. The two maps show similar disposition of the religious groups.
NOTE: The country of Yugoslavia was formed in 1918. Its first name was the "Kingdom of Serbs, Croats and Slovenes". Those three nations were the constituent nations of Yugoslavia. Thus, they have the right to (using peaceful means) negotiate leaving the union.
Albanians of Kosovo are NOT constituent nation of Yugoslavia. They are minority in the true sense of the term. To make a precedent and give Albanians of Kosovo "right" to secede would open a whole new Pandora's box in the international relationships.
[ Ethnic maps of Yugoslavia ]
[ Communist Yugoslavia ]