Study of similarities and differences in behavioral organization among living beings. The discipline pays particular attention to the psychological nature of humans in comparison with other animals. It began to emerge in the late 19th century and grew rapidly in the 20th century, involving experimental studies on human and animal brain function, learning, and motivation. Well-known studies have included those of Ivan Pavlov on conditioning in laboratory dogs, those of Harry Harlow (1905–81) on the effects of social deprivation in monkeys, and those of various researchers on language abilities in apes.

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Scholarly study of the world's musics from various perspectives. Although it had antecedents in the 18th and early 19th centuries, the field expanded with the development of recording technologies in the late 19th century. The term ethnomusicology was introduced about 1950, and the field subsequently became standard in academic institutions. Some ethnomusicologists consider themselves allied with musicology and others with anthropology. Later areas of concern include the study of popular musics as reflections of political, social, and economic movements.

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Economic theory first advanced by Robert Torrens and David Ricardo that analyzes international trade in terms of differences in relative opportunity costs. The theory suggests that countries should specialize in the goods they can produce most efficiently rather than trying for self-sufficiency and argues strongly in favour of free international trade.

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