According to a report released by Standard & Poor’s, the credit ratings agency, only six industrial companies retain AAA ratings in the US, compared with a peak of 32 in the 1980s.

Only two of the current six, ExxonMobil and General Electric, have been steady over that period, and most have been going constantly down the credit ladder.

“Not only do companies not go back, they seldom stop their slide at the next-lowest category in the credit spectrum,” said Nicholas Riccio and Giovanni Galeotafiore, authors of the report.

In 1980, two-thirds of S&P’s corporate ratings were at the investment-grade level, but by the end of the decade the balance had changed and two-thirds were high-yield or junk status. Since 1980 there have been only two years when the number of upgrades exceeded that of downgrades, and today most companies are concentrated in the B category.

Mr. Riccio said many companies had found life a few notches down the credit ladder sufficiently flexible to suit both share and bond holders.

“In 1980, a BBB rating was seen almost as a stigma. Now, companies seem to be aspiring towards BBB or sometimes A to give them the balance they want in dealing with shareholders and lenders,” he said.

The report concludes that a situation was influenced mainly by changing attitudes towards credit risk.