U.S. doctors' real incomes have dropped by 9 percent in the last decade, while a sluggish economy has prompted more physicians to join group practices or set up in smaller towns, an American Medical Association report says.

The report, to be submitted this week to the AMA's annual convention, cites a recent study stating that doctors' annual incomes, adjusted for inflation, dropped to $38,000 from $41,800 between 1970 and 1980.

The study also found that in the last decade:

* The number of doctors per person in the United States increased from about 152 per 100,000 civilians in 1971 to 194 per 100,000 in 1980.

* Office visits dropped by nearly 10 percent for an average doctor while hospital visits fell 18 percent.

* More doctors than ever are in group practices, 88,000 in 1980 compared with 40,000 in 1969.

Dr. Joseph F. Boyle, chairman of the AMA's board of trustees, said doctors' salaries have been declining for several years. Hospital visits have dropped, he said, because, "there's been a concerted effort to do things in a less expensive setting." Boyle said the economy was creating another trend, a dramatic increase in the number of group practices. For an average young physician finishing school, "having the finances to set up private practice alone would be very, very difficult," he said. A physician who has just completed his residency has an average debt of between $40,000 and $80,000, he said.