If you want to know why mortgage rates have stayed so high, despite significant improvements in the United States economy this year, consider the following underpublicized factors: fear, herd psychology, and molases-like response time.

Walter Wriston, chairman of Citicorp and Citibank, said in an interiew last week that interest rates are now "unhooked from the economic realty of the country."

The reality, in Wriston's view, is that the rate of inflation has ben solidly in the single-digit range for most of 1981. Economic reality is also that Congress and the Reagan administration no longer are simply talking about restraining federal spending They're doing it.

Since a major component of interest rates is the premium lenders tack on to their return for anticipated inflation, Wriston believes the money market has underreacted to good news on the inflation front. He thinks rates should be at least several percentage points below whre they are this month.

So, too, do some of the nation's top mortgage-market economists. The "fundamentals" of the U.S. economy would normally justify lower rates -- 14 percent or 14 1/2 percent, rather than 16 1/2 percent to 17 percent, they concede. But the long-term credit market is "skittish, uncertain about Reagan and the spending cuts, and just plain worried about getting burned," in the words of James Christian, chief economist for the U.S. League of Savings Associations.

In short, the "market" -- thar collective, impersonal and uniquely blameless national controller of the cost of money -- is still nervous. Were it less nervous, you'd be getting lower quotas when you call up your local lender for a new mortgage or to refinance your present loan. That's an ecomomic fact.

Were the "market" less hesitant to believe that inflation is indeed controllable, it would be contributing far less to the perpetuation of high rates of inflation in credit-starved industries like housing Mortgage and housing costs represent 20 percent of the Consumer Price Index. Even a modest drop of one or two percentage points in both short-term construction loan rates and conventional long-term home mortgages would pulse through the CPI and help push it lower into the single digits.

That, in turn, could provide justification for still further cuts in rates, and the deflationary cycle might get seriously underway.

But the herd isn't moving.

When might it begin? That's a question best put to psychologists, but here's what two prominent mortgage market economists expect:

Thomas Harter of the Mortgage Bankers Association of America thinks we've seen the interest rate peaks for 1981, and "the road from here on in should be gradually downward" toward 13 percent home loans by the end of the year.

Harter emphasizes the word "gradually." He says that in addition to being nervous about the Reagan administration's prospects for controlling inflation, the long-term capital inflation, the long-term capital market faces unusually heavy demand for its funds, even at record-high interest rates.

The demand comes from corporate and municipal bond issuers -- big businesses and cities of all sizes -- who are poised to grab the capital that could otherwise go to home buyers. They've been frozen out of the market for months because of high rates, and they're willing to pay more than home buyers.

As a result, Harter looks for conventional mortgage rates to be no lower than 14 1/2 percent to 15 percent by October.

Christian says he's "optimistic" that conventional rates will fall to 14 percent later this year, but he thinks that many S&Ls and banks should be able to offer lower rates than that on adjustable-rate "wrap refinances" of existing home loans in their portfolios.

The ball game for prospective purchasers of existing homes by later summer and early fall, according to Christian, may well be in the adjustable-rate arena.

Many S&Ls will begin offering mortgage plans that carry special discounts for new buyers of houses originally financed with the S&L's own low-rate, fixed-payment mortgages, he predicts. For example, a $100,000 house with a $40,000 non-assumable mortgage at 9 percent on it -- vintage 1977 -- might qualify for a 12 percent or 12 1/2 percent adjustable-rate "wrap" refinancing this fall.

The lender would take the $40,000 low-rate loan off its books and replace it with a new $80,000 loan carrying an adjustable rate that starts two or three percentage points below the regular market rate. The $40,000 balance on the original non-assumable loan would be left intact -- as if it had been assumed in a regular wrap-around transaction. The S&L, however, would in fact only lend out $40,000 in new money, thereby conserving its scarce capital and raising the yield on its portfolio.

Adjustable-payment, refinanced loans at 12 percent or 13 percent may not to be everybody's idea of a bargain in the mortgage market. But if Harter's and Christian's forecast on rates for the rest of the year are accurate, loans like that will be just about the best the market has to offer.