Over the past decade or so, much of the Western world has succumbed to a strange and apparently lethal new disease. It is a disease of the mind, a psychological obsession, surrounding the nation and economy of Japan: part phobia, part philia; drawing both from Japanese and Western intellectual tributaries; combining obscure racial theory, tendentious economic analysis, and misty sociological observation; at once self-loathing and self-satisfied; popularized by repeated journalistic comment, but always emphasizing the economic superiority and inevitable triumph of the Japanese way.

There is, of course, much reason to admire Japanese economic performance. From 1960 to 1987, real gross domestic product per head grew by an average of 6% per year, more than twice the rate of any other leading industrial country. Why have other nations, such as our own, not matched this performance? In the Japan-obsessive view, this can only be attributed to a failure of nerve or ideological blindness on the part of our leaders. The possibility that Japan's economic rise in this century might simply involve the more rigorous application of the same principles underlying our own previous miracles - high savings rates, flexible labor markets, sound money, competitive markets, and a predominantly middle-class social structure - is not entertained.

PET CAUSES

The persistent habit of ascribing perfectly explicable economic phenomena to miracles worked by MITI, or superhuman workers, or Buddhist management principles feeds the preconceived theories of those looking to rationalize their own pet causes abroad. Everyone finds in Japan's supposed omnipotence what they want to find. Some see the ''Japanese model'' as something to be emulated, urging speedy adoption of the central planning on which it is supposedly based. Others, seeking protection for Western industries, decry ''unfair'' Japanese practices, whether government policies or just plain habits of life. But both agree that Japan has pretty much got the drop on us.

This latter kind of nonsense has reached an alarming pitch in the U.S., where Japan's economic policy is habitually described in terms of ''adversarial trade.'' James Fallows writes in this month's Atlantic of the need to ''contain'' Japan, for its own good as well as that of the U.S. Less sympathetically, Time asks: ''Does Japan play fair?'' and concludes, unsurprisingly, that it does not, quoting in support such ''dedicated free traders'' as Chrysler Chairman Lee Iacocca. In the most discreditable version yet, Murray Sayle, noting also German industrial might, wonders in the current issue of the New Republic whether ''the Axis is back in business, pursuing the old aim of weltmacht by subtler, more seductive economic means?''

There are several articles of economic prejudice to subscribe to before one can even begin to approach this level of dread at the Japanese threat. You have to believe, first, that there is a threat: that trade is a form of economic warfare, in which one country's success can only come at the expense of another's. You have to believe that a Japanese trade surplus is an indication of some fundamental disparity in competitiveness, rather than a necessary reflection of capital flowing in the opposite direction.

You have, indeed, to swallow a lot of basic factual errors. If there is any economic contest going on, Japan's economy has not ''won'': it's not even close. Overall productivity in miraculous Japan is about the same as in France. It would have to be 30%-40% larger to match the North American economies. Japan's economy is not export-led: exports take about 10% of gross national product, not far off U.S. proportions. Nor is it particularly import-shy: tariffs are at roughly American levels, overt nontariff barriers are fewer, and contingent protectionism of the ''Super 301'' variety is all but unknown. Overall, imports account for about the same share of the its economy as the average for developed countries, adjusting for the size of the economy (since big countries tend to import less).

All of the discussion of Japan's alleged pathological tendency to trade surpluses is intended to drape smart new clothes on a very old American idea: reciprocity. This made no economic sense when Alexander Hamilton proposed it, and it makes even less now. A country can liberalize trade on its own and benefit, without meddling in other countries' affairs.