Robert Ader, 79, an experimental psychologist who was among the first scientists to show how mental processes influence the body's immune system, a finding that changed modern medicine, died Dec. 20 in Pittsford, N.Y.

His death followed a long illness and complications of a fracture suffered in a fall, his daughter Deborah Ader said.

Dr. Ader, who spent his entire career as a professor of psychiatry and psychology at the University of Rochester School of Medicine and Dentistry, conducted some of the original experiments in a field he named himself, psychoneuroimmunology.

His initial research, in the 1970s, became a touchstone for studies that have since mapped the vast communications network among immune cells, hormones, and neurotransmitters. It introduced a field of research that nailed down the science behind notions once considered magical thinking: that meditation helps reduce arterial plaque; that social bonds improve cancer survival; that people under stress catch more colds; and that placebos work not only on the human mind but also on supposedly insentient cells.

At the core of Dr. Ader's breakthrough research was an insight already obvious to any grandmother who ever said, "Stop worrying or you'll make yourself sick." He demonstrated scientifically that stress worsens illness - sometimes even triggering it - and that reducing stress is essential to health care.

That idea, now widely accepted among medical researchers, contradicted a previous principle of biochemistry, which said that the immune system was autonomous. As late as 1985, the idea of a connection between the brain and the immune system was dismissed in an editorial in the New England Journal of Medicine as "folklore."

"Today there is not a physician in the country who does not accept the science Bob Ader set in motion," said Bruce Rabin, founder of the Brain, Behavior and Immunity Center at the University of Pittsburgh Medical Center, who considered Ader a mentor. "He attracted interest in the field and made it possible to prove that 'mind-body' is real."

Dr. Ader was born in New York. His father, who owned a wholesale liquor company, died in a car accident in 1945 when Robert was a teenager. After graduating from the Horace Mann School, he earned his bachelor's degree from Tulane University and, in 1957, his doctorate in psychology from Cornell.

Soon after, he became an assistant professor in the department of psychology at the University of Rochester, where he went on to hold many teaching and research posts. He retired in July as a professor emeritus of psychosocial medicine. - N.Y. Times News Service