Dr. John G. Clark, a Harvard psychiatrist whose study of new religious sects in the 1970s raised public awareness of the overwhelming influence of some groups over their members, died on Oct. 7 at a nursing home in Belmont, Mass. He was 73 and had been suffering from a long illness, his family said.
Clark immersed himself in the study of new or generally unfamiliar sects like the Unification Church, the Church of Scientology and the International Society for Krishna Consciousness. Over time, he counseled more than 500 former members of the groups and their families.
Dr. Margaret Thaler Singer, an expert on such groups and an emeritus professor of psychology at the University of California at Berkeley, said Clark was among the first professionals to turn attention to the subject, even before the mass suicide of People's Temple members in Jonestown, Guyana, in 1978.
The following year, Clark wrote a widely cited guest editorial in The Journal of the American Medical Association warning about the growing power of such religious groups. He was also called as an expert witness before a congressional committee investigating religious sects. "People paid attention," Dr. Singer said.
In his editorial, Clark wrote, "The new youth cults, though usually self-styled as religious for purposes of First Amendment privileges, are increasingly dangerous to the health of their converts and menacing to their critics."
At first, Clark himself had to be convinced of what was then not a generally accepted principle: that an ordinary person, not suffering from any significant psychiatric problem, could within a matter of days be persuaded through simple group pressures to walk away from a previous life and devote everything to a particular group.
"I think, like most psychiatrists, initially he was rather skeptical," said Dr. Michael Langone, executive director of the American Family Foundation, a 20-year-old organization that works against such sects. Clark was active in the organization.
But Clark's views changed as he met with more patients.
"Orthodox psychiatric opinion has generally viewed conversion to deviant groups as a function of longstanding conflicts within individuals," he said in a 1982 interview. "Our evidence strongly suggests that these individuals are succumbing to pressures within the cult milieu, pressures that can induce radical personality changes as easily in normally developing people as among disturbed ones."
Through the 1980s, Clark was called upon by the news media, families and psychiatrists for his expertise on the influence of sects.
The Church of Scientology objected strongly to Clark's assertions, and the church and Clark battled in court. Clark said the church had engaged in a campaign of harassment against him. In 1988, he settled with the church and received an undisclosed amount of money, but agreed never to discuss the group publicly again.
John Gordon Clark was born on March 3, 1926, in St. Cloud, Minn., one of three boys who grew up to become doctors.
Clark graduated from Harvard Medical School in 1953 and became an assistant clinical professor of psychology some years later. He worked on the staffs of Massachusetts General and McLean Hospitals while at Harvard. He continued to see patients through the early 1990s.
Clark is survived by his wife, the former Eleanor Sherwood of Weston, Mass.; two brothers, Robert of Minneapolis, and Bruce of Tucson, Ariz.; a daughter, Catherine Clark of Waltham, Mass., and a son, Gordon, of Springfield, Vt.