K David Harrison writes:
“How many ethnic groups are there in the world? A complete list of them all would have well over 10,000 entries, and perhaps many more. The exact number is unknown and ever changing. Ethnicity itself remains an elusive concept, and there is no solid, universally accepted definition of what constitutes an ethnic group.
In typical usage, the term denotes a group of people who strongly identify themselves (or are identified by others, even against their will) as belonging together based on specific common traits they share. Such traits are largely involuntary: for example, skin color, clan or tribe membership, perceived or actual common ancestry, shared history, language, disability (e.g., deafness), or sexual orientation.
Other traits that group people into ethnicities may be chosen, abandoned, learned, or changed. These include culture, religion or sect, age, caste or social status, speech dialect, place or means of habitation, way of life, marriage into a group, immigration and naturalization – and on and on. Throughout history, people have imagined and formed communities in order to survive, to thrive, to have social support, and to realize their full potential. Many such communities are viewed and defined as ethnic groups.
Language is perhaps the most salient and typical marker of ethnicity, but there are exceptions. The Baraguyu people of Kenya speak the Masai language, but they consider themselves to be a separate ethnic group from the Masai. Many speakers of Chinese cannot understand each other at all, yet they may feel that they all belong to the Han Chinese ethnic group.” (p.358)
“As we move toward a deeper appreciation of human diversity, we embrace more flexible and less rigid notions of ethnicity. Many people have come to appreciate their mixed or multiethnic status as an asset, not a disadvantage, and resist being assigned to a single ethnic category. People all around the world can legitimately identify as a member of more than one ethnic group, depending on the context.” (p.358)
“Though we may be moving toward a more flexible notion of ethnicity, dark forces such as nationalism, ethnic chauvinism, and even ethnic cleansing work to reinforce rigid barriers that divide ethnicities. Ethnicity in itself is neither good nor bad. It can be a source of comfort, pride, and belonging.” (p.358)
Ref: Eds. Wade Davis and K David Harrison with Catherine Herbert Howell (2007) Book of Peoples of the World. National Geographic: Washington DC