A 42-year-old unmarried New Jersey salesman with lifetime average earnings of $12,000 a year has a heart attack at home one night. It results in permanent and total disability.

Under federal and state laws for the support of the disabled, the man will receive long-term disability income payments from Social Security totalling $424 a month.

Suppose, however, that he had been disabled in an auto accident on the job. Because the accident involved an auto, he'd also get no-fault auto insurance disability payments. And because he was on the job when it happened, he'd get workers' compensation.

His total, in a typical accident of this type in New Jersey, would be $1,433 a month in combined payments - far more than in the first case and more than he actually earned in gross pay before disability.

And if he also held a privately purchased disability insurance policy, he could get even more.

These illustrations were computed by the Research Subcommittee of the Health Insurance Association of America in attempt to show that the disability insurance "system" in the U.S. is a hodgepodge of overlapping programs - in some cases leaving broad gaps, in others allowing an ill worker to pyramid different benefits into a far greater total than he earned before.

Benefits that overlap or are in excess of prior earnings result in excessive claims and encourage people to stay on disability after they could become rehabilitated, according to the report.

Without spelling out specific steps, the report said some method of making the various programs - including private insurance - complement each other and coordinate with each other would be desirable to end waste and inducements against returning to work.

The report noted that there is already some coordination: Under existing law, combined payments from state workers' compensation policies and Social Security disability can't exceed 80 percent of prior income.

But the report said many other programs that pay cash support on disability aren't always fully integrated into any single benefit limit: There are veterans' benefits, group disability insurance financed by employers, state cash disability sickness payments, no-fault, privately purchased disability policies, and a whole pack of other public programs covering special groups of the population such as longshoremen and harbor workers and state and local employes.

The insurance group said cash benefits equal to anywhere from 55 percent to 65 percent of previous gross income generally should be sufficient to support a disabled person in addition to medical insurance. (Some groups dispute this and argue that the figure should be higher - 75 percent or perhaps more.)

The insurance group said its own studies show that when cash benefits top 70 percent, claims rise sharply - in one study to 199 percent of the expected level, in another to 219 percent.

And Paul Barnhart, president of the Society of Actuaries, said in a telephone interview from St. Louis that a big rise in recent years in outlays for private disability insurance benefits by companies is due largely to people staying on the rolls longer after they realize how high their benefits can be.

People don't feign illness in order to get on the rolls, but once on, "they realize only then for the first time how liberal their benefits actually are," Barnhart said recently in a speech.

One study shows that for white collar males age 40 to 49, the number of claims per 1,000 persons insured actually went down very slightly from 1966 to 1975, but the duration of stay on the benefits increased from 1.58 months on the average in 1966-67 to 2.44 months in 1975, Barnhart said.

The reports said these factors - overlapping benefits and disincentive to get off the rolls - help to account for sharp rises in the costs of disability payments in both the public and private programs in recent years.

The report said all federal government systems combined are expected to pay out $25 billion next year, private group policies accounted for $2.8 billion in 1978, state and local government systems $3.1 billion, certain special state cash sickness programs $1 billion, and private individual policies about $1 billion.