Pension fund managers have continued to take an extremely cautious attitude toward stock market investments in 1978, reducing the percentage of equity exposure in their portfolios to below 55 percent.

At the same time, the people who run the hundreds of billions of dollars of future retirement assets for American employes could earn the distinction of outperforming the stock market average this year for the first time since 1971.

Through the first nine months of this year, the rate of return of equity investments for the median fund in A. G. Becker's data bank of 4,000 nontaxable funds registered a 13.2 percent gain compared with an increase of 12 percent for the Standard and Poor's list of 500 stocks.

But the gain may not hold for the year. In the third quarter, the median fund registered an 8.3 percent gain compared with an 8.7 percent increase in the S and P 500. And the tricky stock market experienced so far in the fourth quarter also could put the managers behind.

The survey by A. G. Becker, which is three quarters pension funds, is considered the most up-to-date guideline of pension market performance available.

According to the tabulation, pension funds had committed 54.4 percent of their assets to equities in the third quarter, down from 56.6 percent at the end of 1977, and compared with a rate of more than 70 percent in 1972 before the ratio started to plunge.

The swing from stocks to bonds that has been taking place in the interval since 1973 was curtailed somewhat as a declining bond market caused pension fund managers to reduce the proportionate commitment to long-term bonds as well as stocks this year.

At the end of September, 23.7 percent of pension fund assets were committed to long-term bonds versus 25.8 percent at the end of last year.

Instead, fund managers have taken advantage of the high interest rates available in short-term money instruments and parked a growing percentage of the assets there for the time being.

"What has happened is the liquid cash reserve position has gone up significantly from the beginning of the year," observed Robert Demichele, eastern regional manager for Becker's retirement funds evaluation group.

The percentage of pension funds in cash equivalents rose to 13.7 percent at the end of this year's third quarter, compared with 10.8 percent at the end of 1977.

"What was more significant as we looked at calendar 1977 was that it was the first time in 10 years that the median committed to common stocks dropped below 60 percent," Demichele said. "And that has gone down even further in 1978, though this may not be so much a function of selling of equities as of money not being committed to securities."

Pension fund managers, in effect, have a problem with a rapid build-up of investable funds and no good place to put them.

This year alone, pension assets for all funds, including corporate, private and government, are expected to increase by a staggering $60 billion from their estimated $500 billion base. That represents a 12 percent increase, which is actually an acceleration in the 10 percent growth rate that pension assets have registered in the past few years.

If this 12 percent rate of increase continues, it would double pension assets in all funds to $1 trillion in only five years.

As it is, only a one-percent change in the amount allocated to any category of investment by the pension funds today represents $5 billion.

This means there is immense reserve buying power available that could fuel a bull market in stocks if and when pension funds ever decide to increase their equity positions.

But so far this year the trend has been in the opposite direction.

"In the first quarter, private sector pension funds withdrew about 3 percent from the market; in the second quarter there was zero change. I believe they were sellers on lanace in the third quarter, and I think they are still doing it," said Harold Ehrlich, chairman of Bernstein-MacAulay, a New York investment adviser that manages about $1 billion in stock market funds.

Ehrlich cited a number of reasons for the continuing decline, including the poor performance record of fund managers over the last decade that has seen them lose money in sic of the last ten years.

"They are scared of their money in fixed income investments to make sure there is at least some rate of return," said Ehrlich.

The record of fund managers earlier this year in outperforming the market averages has made them anxious lest these gains erode in the final period, so "some managers are in the process of selling to nail down these gains to make sure the gain for the whole year doesn't disappear," he added.

And the clients are meanwhile exerting pressure on the manager to recuce their equity commitments to about 50 percent, he indicated.

"At some point this reserve buying power is going to flow into common stocks, and as I look ahead two or three years I see a tremendous bull market coming," said the Bernstein-MacAuley chairman. "I don't know when it's coming, but it will."

Other fund managers concur that at some point there will be a clash of enlightment revealing the many good values to be obtained in the stock market which will spur a buying stampede by the pension funds. But for now, the big pension funds have used any strength in the market this year as an opportunity to sell rather than buy.

"We've used periods of strength to reduce our exposure in equities," said Edward Hunce, who is in charge of the largest of the pension fund managers.

"One problem has been that while we recognize the uncertainty caused by some of the administration's policies, and that interest rate and inflation pressures were going to make stocks questionable holdings in the near-term, that stock at the same time seem generally undervalued for the long-run," said Bunce.