To classify humans on the basis of physical traits is difficult, for the coexistence of races through conquests, invasions, migrations, and mass deportations has produced a heterogeneous world population. Nevertheless, by limiting the criteria to such traits as skin pigmentation, color and form of hair, shape of head, stature, and form of nose, most anthropologists historically agreed on the existence of three relatively distinct groups: the Caucasoid, the Mongoloid, and the Negroid.
The Caucasoid, found in Europe, N Africa, and the Middle East to N India, is characterized as pale reddish white to olive brown in skin color, of medium to tall stature, with a long or broad head form. The hair is light blond to dark brown in color, of a fine texture, and straight or wavy. The color of the eyes is light blue to dark brown and the nose bridge is usually high.
The Mongoloid race, including most peoples of E Asia and the indigenous peoples of the Americas, has been described as saffron to yellow or reddish brown in skin color, of medium stature, with a broad head form. The hair is dark, straight, and coarse; body hair is sparse. The eyes are black to dark brown. The epicanthic fold, imparting an almond shape to the eye, is common, and the nose bridge is usually low or medium.
The Negroid race is characterized by brown to brown-black skin, usually a long head form, varying stature, and thick, everted lips. The hair is dark and coarse, usually kinky. The eyes are dark, the nose bridge low, and the nostrils broad. To the Negroid race belong the peoples of Africa south of the Sahara, the Pygmy groups of Indonesia, and the inhabitants of New Guinea and Melanesia.
Each of these broad groups can be divided into subgroups. General agreement is lacking as to the classification of such people as the aborigines of Australia, the Dravidian people of S India, the Polynesians, and the Ainu of N Japan within the traditional three race system. These exceptions highlight the problems associated with attempting to classify humanity into races and also challenge the validity of the notion of race when applied to human beings.