Individual investors, who fled the stock market during the first half of the 1970s, came back strongly during the last few years, according to a study released today by the New York Stock Exchange.

Most of the new investors are younger, less affluent and own less stock than individual investors who have been in the stock market for a number of years.

Despite the surge in stock ownership during the last few years, fewer investors own stock themselves today than in 1970. The 29.8 million stockholders in 1980 represent a 4.6 million increase over 1975, but they are 1 million fewer than in 1970, according to the New York Stock Exchange study. Probably another 115 million individuals have an indirect share in the stock market through their pension funds or life insurance policies.

Although the number of stockholders has grown, the amount of stock individuals own is smaller than five years ago.

The New York Stock Exchange conducts a survey of individual stockholders every five years.

William M. Batten, chairman of the exchange, said the "resurgence of corporate ownership in America" is "good news." In 1975, the exchange reported the first decline in stock ownership since the NYSE began the survey in 1952. He attributed the increase in stock ownership to the dramatic improvement in stock prices in the last two years and to the liberalization of taxes on capital gains that went into effect in 1979.

The survey reported that there are 148,000 shareholders in Washington, D.C., 54,000 more than in 1975 but about the same as a decade ago. About 658,000 Marylanders own stock -- up from 560,000 in 1975 but fewer than the 715,000 Maryland residents who owned corporate shares in 1970. The survey estimated that 667,000 Virginians owner stock in 1980, up 31,000 from 1975 but 27,000 fewer than in 1970.

Since 1975, some 6.9 million citizens have become "shareowners for the first time -- more than offsetting the 1.9 million who died or stopped being shareowners during that period," according to Batten. The average new shareowner has a portfolio of stocks worth about $2,000 and is 36 years old, while the long-time investor is 51 years old, with a portfolio worth substantially more.

As a whole, the 29.8 million shareowners have a median age of 45.5 years and own about $4,000 worth of stock. Five years ago, half the stockowners were older than 52.5 years, and the average owner had a portfolio worth $10,100.Half of the 1980 stockholders went to school 15 years or more, about the same as in 1975.

The median income for individual stockowners is $27,750, which is 46 percent higher than in 1975, an increase that is consistent with a 48 percent rise in the household income of the population at large, although the population at large has a substantially smaller household income of $16,550.

"Given the rising stock markets of the last couple of years, the survey findings relating to stock portfolio size may seem inconsistent," according to Stan West, NYSE vice president for business research. He said several factors have caused the decline in median holdings from $10,000 to $4,000, including:

The increase in younger individuals, who typically make smaller investments than existing shareholders.

Bigger increases in holdings of stocks traded over the counter and on the American Stock Exchange. These stocks are generally about half the price of the stocks traded on the bigger New York Stock Exchange.

A sharp rise in the use of employee stock purchase plans, where smaller acquisitions of stock generally are made.

"Persistent partial liquidation of stock portfolios for investment in new vehicles which were virtually nonexistent in 1975, such as money market funds and tax-free municipal bond funds."

Most shareowners live in large metropolitan areas, half of them in areas with populations of more than 1 million.Shareowners in nonmetropolitan areas have shrunk from 20 percent of the total in 1975 to 16 percent today.

Male shareowners outnumber females slightly -- 14 million adult men and 1.5 million minor males own stock compared with 13.5 million adult women and 828,000 minor females.

California remains the state with the most shareholders -- 3.72 million -- but New York City edged out of Chicago for the metropolitan area with the most shareholders. Although Chicago is about half the size of New York, the Windy City traditionally has been the metropolitan area with the most number of individual stock investors.