What many older investors do not know about how stockbrokers are paid could hurt them, according to a survey by the nation's largest lobbying group for seniors.

While most older investors know that brokers are paid in commissions, they have a limited understanding of the details of brokers' compensation and how that could affect the advice they get, according to the new survey by the American Association of Retired Persons.The AARP warned that seniors' lack of knowledge may leave them unaware of potential conflicts of interest that could make brokers work against them.

Women aged 50 and over in particular "have serious gaps in their understanding of the (broker) compensation system," said the study based on interviews with 827 people in that age group who own securities.

The survey came just days after Arthur Levitt Jr., the Securities and Exchange Commission chairman, threatened to clamp down on broker pay practices - such as higher commissions for higher-risk investments - that can create conflicts of interest.

In 1995, Levitt urged the securities industry to change such practices voluntarily. But his frustration with the industry's response led him to threaten to impose new regulations.

"We simply must accelerate efforts to reduce or eliminate con-flicts of interest between brokers and their clients," Levitt told a securities industry group Monday in Scottsdale, Ariz. Excessive incentives for brokers "can create strong pressures not only to sell but to sell anything," he said.

Among the AARP survey's findings:

- Only about one in three older investors (36 percent for both men and women) know brokers often get higher commissions for selling higher-risk investments.

- More than a third (39 percent) do not know their initial investment is reduced by the broker's commission. That represented 47 percent of women investors and 33 percent of the men.

- More than a third (37 percent) do not know the term "load" refers to a sales charge. For women it was 53 percent; for men, 25 percent.

- Nearly half (48 percent) do not know customers can negotiate the commissions, including 61 percent of women and 39 percent of men.

- One-third (33 percent) do not know some firms use contests to promote sales of particular investment products. For women it was 40 percent; for men, 27 percent.

The National Association of Securities Dealers, a self-regulatory group, has recommended that such contests be curtailed.

The survey, conducted by International Communications Research of Media, Pa., polled 827 people from Nov. 14, 1997, to Jan. 4, 1998. The margin of error was 2.1 to 3.5 percentage points, depending on the question.