The headlines tell the story:

The Post, Nov. 1 -- "GM Reports $2 Billion Loss, Plant Closings -- Automaker Moves to Reduce Capacity in Face of Slow Sales."

Fortune Magazine, Nov. 19 issue -- "Toyota -- Why It Keeps Getting Better and Better and Better."

The growing strength of the Japanese auto industry, symbolized by Toyota's success, mirrors the decline of the American industry, symbolized by General Motors' painful effort to shrink itself to a lesser role.

In the good old days, GM was in such control that in order to steer clear of potential monopoly charges it deliberately tried to avoid selling more than 50 percent of the market. Now its share of sales has plunged to 35 percent -- if you include GM's Geo line of imports, which account for 2 percent.

The steady gains by Japanese automakers, by contrast, have boosted their U.S. market share to 31.5 percent, counting Japanese nameplates produced in factories in this country. (On the West Coast, Japanese cars have more than half of the total market.)

Often, Detroit's sad condition is cited as a prime example of how Japan destroys a key American industry. But there is every evidence that General Motors (and Ford and Chrysler, which began a slimming-down process earlier) have only themselves to blame for losing out to imports.

The overwhelming reason for Japanese success is proven quality, performance and styling that Detroit, despite some effort in the right direction, hasn't been able to match. A study to be released next month in the Brookings Papers series will report that brand loyalty, especially to General Motors cars, has eroded so seriously in the past decade that the situation cannot be turned around overnight.

Even if American drivers were convinced that American car quality matches Japanese quality (which they are not, as of now), it could take another 10 years to stop the plunge in American market share, the study suggests.

Clifford M. Winston of the Brookings Institution staff and Fred Mannering of the University of Washington find that brand loyalty depends on cumulative experience of customers over years, not on improvements in current models.

Thus, it does Chrysler boss Lee Iacocca little good to assert in TV commercials that American cars are better than Japanese. Even if Iacocca's claim were true, car-buyers tend to look at the record -- and they have long memories. "The Japanese cars have the momentum of history behind them," Winston says.

Consumer Reports, the influential advisory service, has repeatedly warned buyers that new GM models are generally "troublesome" in their first year, according to Bob Knoll, head of CR's auto test division. "Unfortunately," Knoll says, "GM much of the time hasn't improved {its new models} a great deal."

A classic case illuminating the depth of the problem for American car makers is the experience at the joint GM-Toyota operation in Fremont, Calif. Some vehicles roll off the assembly line with Chevrolet's nameplate, "Geo Prism." Others roll off as Toyota Corolla. It's the identical automobile -- but in the marketplace, the Toyota Corolla commands a substantial price premium over the Chevrolet.

Looking at the American industry as a whole, almost all independent observers agree that quality is now better, in response to competition from Japan. Nonetheless, Japanese companies manage to keep a step ahead. Fortune's story on Toyota, for example, rates it "the best car maker in the world," a company whose "success never gets in the way of constant improvement." Fortune notes that Toyota this year has improved its subcompact Tercel, which still sells at last year's price: $100 cheaper than GM's vaunted new Saturn and as much as $1,600 less than some other competitors' models.

Detroit's self-inflicted wounds go beyond its inability to match Japanese quality. When "voluntary" quotas were established, limiting the number of Japanese cars that could be imported, Detroit had to choose between two strategies: (1) lower prices to increase market share, and (2) raise prices regardless of the effect on domestic output.

American auto moguls exercised the second option, maximizing prices and profits but cutting total demand for American cars. Thus, the United Auto Workers union -- which backed, even demanded, the quota concept -- found that jobs had been lost for American workers. The industry had opted for the short-run maximization of profit even though it would result in the loss of market share to Japan. And that market share, given the high brand loyalty attaching to Japanese cars, is proving hard, if not impossible, to win back.

A parallel blunder for which Detroit continues to pay was its response when the Japanese yen began to rise against the dollar, forcing Japanese car prices higher. That circumstance provided another golden opportunity to hold down American prices, creating a favorable differential that might offset the imports' advantage in quality.

Alas, Detroit followed Japanese prices higher, again sacrificing market share for quickie profits. "They blew it," says Winston.