WASHINGTON— WALK through the Freer Gallery on Washington's Mall these days and you will run across an exhibit showing the advances in Japanese art that came 400 years ago with the development of a merchant class. Walk a little farther, and there is a remarkable exhibit of Whistler's century-old paintings, testimony to what wealthy patrons can give to society.

But while high concentrations of wealth may have served culture, a growing body of research suggests that big gulfs between the affluent and the poor tend to result in more social unrest, slower economic growth and investment and lower standards of living.

The conventional wisdom used to be that the disparity between rich and poor was a price of economic growth, particularly for poor countries pursuing industrialization. Differences in income were perceived as necessary to provide an incentive for people to strive. And without those people who earn considerably more than they need for life's daily necessities, a society may not have enough savings to pay for new factories, equipment and the other underpinnings of an industrialized economy.

But that conventional wisdom has been challenged in recent years as economists have noted that countries with relatively equal distributions of income have achieved higher rates of economic growth than those with considerable economic inequality. In the more unequal nations, economists argue, more people lack access to the education and training that would enable them to contribute to economic growth.

These concerns were underlined by a study released last week by a nonprofit research institute in Luxembourg that found poor children in the United States to be poorer than the children in 15 other Western industrialized nations. Only in Ireland and Israel were the poor children worse off.

The study followed several academic papers released last spring that say the United States has the most unequal distribution of income and wealth of any Western industrialized country. While many middle-class families have become very affluent as the economy has expanded over the last 15 years, many other families are sinking financially, so that the ranks of the middle class are thinning. The result has been alarm among many economists.

"I don't know what the right level of inequality is," said Timothy M. Smeeding, one of the authors of this week's study on child poverty. "I think the American level is too great."

Especially provocative is a study by Alberto Alesina at Harvard University and Roberto Perotti at Columbia University that is scheduled for publication soon. After examining statistical patterns in 70 countries, the two economists concluded that high income inequality creates social unrest and pressures for heavy taxes on the rich and, in turn, economic uncertainty that discourages investment. With lower investment, economic growth suffers.

More for Fewer

A separate study this summer by Professor Perotti alone concludes that more equal societies also tend to have fewer children and invest more in education -- two trends that foster economic growth. Both studies strongly reflect the experience of poor countries, but also include affluent countries like the United States.

Yet the ways in which equality are achieved seem to be at least as important as the actual gap between rich and poor. East Asian countries that have had relatively equal distributions of wealth dating from before their industrialization have fared far better economically than countries like Russia that leveled their differences by force.

Nor is there any consensus among economists about the extent to which economic inequality may justify extra government taxes and spending to redistribute wealth.

"The ways we have gone about reducing income inequality have had serious inefficiencies," said Robert H. Haveman, an economics professor at the University of Wisconsin at Madison, who cited welfare payments and taxes that may reduce incentives for many people to work.

Even the popular wisdom that economic inequality is good for art is coming under question. While Spanish painters like Velazquez produced magnificent works while serving as court painters in Madrid during the 17th century, painters like Vermeer flourished at roughly the same time in Holland, where there was a larger middle class and painters could support themselves by selling to many clients.

As governments have expanded into an ever broader range of activities in this century, the existence of a wealthy class has become even less important for art. When the Depression laid low many wealthy American families, for instance, President Roosevelt's Public Works of Art Project financed a wide range of high-quality art.

One painting from the period, showing an apparently unemployed worker selling apples for a nickel apiece on a quiet wharf, now hangs in the office of Treasury Secretary Robert E. Rubin, who has nicknamed it "the Reagan Economy." Over lunch last week, Mr. Rubin argued that rising economic inequality was a threat to the nation's social fabric and economic growth, and even the arts.

On Art and Law

Mr. Rubin would maintain Federal spending on education and the National Endowment for the Arts, which Republicans in Congress are trying to curtail. "If you had a better educated lower half of the American income class," Mr. Rubin said, "then you'd have a much broader market for the arts."

The long, slow fall in the nation's median wage, Mr. Rubin said, has led to more crime and fostered political extremism as many Americans feel a dwindling stake in the economic system. Partly as a result, the nation has up to three times as many private security guards as police officers.

The answer, he contended, lies in maintaining spending on education to produce skilled workers who can compete in the global economy. The economic troubles of a broad section of American society "are a very serious threat to our social fabric," Mr. Rubin said. "It feeds the violation of law, and that is a very big productivity cost on the country."