Tests on 50 of the country's top players "revealed signs of memory impairment and slowed ability to process information at speed," said the South African Rugby Football Union medical consultant, Dr Ismail Jakoet.
The research, carried out in the Eastern Cape province, showed that forwards were especially at risk of brain injury because they were most exposed to knocks to the head.
Tests performed on the players, with cricketers in a control group, showed varying degrees of weakness in a range of brain skills, including verbal fluency, hand-motor function, visual perceptual processing at speed and verbal memory tasks, such as repeating numbers backwards and forwards.
Research co-ordinator Professor Anne Jordan, from Rhodes University, said it was becoming clear that knocks to the head eventually reduce the brain's reserve capacity.
"There may be no noticeable effect until another blow to the head or until old age 30 years down the line," she said.Reuse content