Economist finds outlooks, not looks, key to success of ethnic groups; The Economics and Politics of Race: An International Perspective, by Thomas Sowell. New York: William Morrow & Co. 324 pp. $15.95.

''Intergroup differences are real, large, and enduring.'' So says Thomas Sowell, a black economist, whose latest book is a global extension of a thesis he has expressed in several previous works, most notably in ''Ethnic America.'' The argument, simply stated, is that cultural values are the most important bases for the economic successes of ethnic groups.

While not denying that geographic isolation, limited resources, population pressures, and patterns of segregation may severely inhibit social advancement, as is often claimed by writers on racial and ethnic relations, Sowell thinks far too little attention is paid to the inherent characteristics of groups themselves, to the traits that determine their lifeways and which they carry as cultural baggage when they resettle in new environs. His theory has caused some to call him a racist and others to say he is a traitor who plays into the hands of those looking for scholarly legitimization of their own views of the innate inferiority of certain people.

He is neither. Rather, he is a classical conservative who raises a number of nagging questions about the proclivities and performances of particular groups.

Sowell asks why the Chinese, the Japanese, and others, like the Jews - vulnerable minorities, frequent scapegoats, ready targets - have fared so well in America. He says it is because they maintained tight families, held to a highly disciplined work ethic, stressed the importance of education, and pushed their children to be better than themselves. His critics might accept the fact that members of such groups were as described but would also add that they were able to succeed because they weren't members of the truly pariah categories in this Herrenvolk democracy. They weren't black - or brown or red. Sowell would counter that this line of reasoning is belied, not only by the fact that those called ''Orientals'' suffered under policies that were blatantly racist themselves, but by ample evidence of significant differences between black West Indians and native-born ''Negroes,'' the former doing far better in the aggregate and individually in the United States than the latter.

The key variable is not ''looks'' but outlooks. In fact, a careful reading of Sowell's book suggests that it may be mistitled, for it is really about ethnicity, not race, at least not in the purely biological sense. (Even when the term ''race'' is used, the author generally follows the fuzzy convention of many contemporary anthropologists, referring to those who share a common heritage.)

Sowell seeks to provide evidence of and explanations for the differential successes of various subgroups in this country and elsewhere. Part 1, ''History, '' begins with a consideration of the critical significance of group membership and the role of enculturation in determining attitudes, incentives, ways of seizing opportunities, and means of coping with adversity. Case studies of the overseas Chinese, European emigres in Australia and the Americas, and ''Blacks and Coloreds'' in various parts of the world emphasize the interaction of dominant groups and minorities in native lands and abroad.

The four chapters that make up the second half of the book are in many ways more interesting and more uneven than those in the first. In Chapter 5, ''An International Perspective,'' Sowell defines his terms, suggests that stereotypes carry with them more than a kernel of truth, distinguishes between discrimination and exploitation (in a manner that doesn't satisfy this writer), and dwells on the effects and limits of politics and of the market.

Here, as elsewhere in the text, extensive footnoting is set to serve as grounding for the case(s) being made. The sample is a biased one, however, and much is left out, including reference to such important volumes as R. A. Schermerhorn's ''Comparative Ethnic Relations,'' which indicated the interplay of the politics and economics of ''race'' 15 years ago.

A chapter on ''The American Experience'' is a fairly straightforward summary of what was said in Sowell's ''Ethnic America.'' The next one, on ''The Third World,'' is different. It is a curious and evocative mixture of points and counterpoints, commentaries and critiques, and challenges to those who would infer causation based on the correlative occurrence of two or more phenomena (poverty and population density being the case in point).

At the end, Sowell repeats many earlier arguments and restates the central theme: ''In short, group separation is not something arbitrarily imposed from above by 'society.' It is a widespread human phenomenon through history and around the world.'' He also worries about the politicization of economic and social life, which, as he sees it, merely exacerbates mutual hostility.

Such concerns are very real and need to be thoughtfully considered by all who care about human relations. So, too, do Sowell's controversial conclusions about success and failure.

If Sowell is correct in his assumptions, then how, in his own words, can we ''make (history's) experience the basis for a better today and a better tomorrow?

He doesn't say.