Three experts, speaking at a School of Public Health forum, yesterday debated the causes of homelessness, faulting the federal government, state mental hospitals and the housing market for the growing phenomenon.

The speakers--a psychiatry professor, a state human services official and the director of the state's homeless coalition--disagreed on Massachusetts' effectiveness in combatting the problem, but agreed the extent of homelessness is impossible to pinpoint.

To begin solving the homelessness problem, the government should emphasize "better support in the community" to help the homeless recover their "affiliative bonds" with other people, William R. Breakey, director the community psychiatry program at the Johns Hopkins Medical School, told the 100-person audience.

Breakey also said that "homelessness and mental illness are intertwined."

Another speaker disagreed with Breakey's emphasis on community support.

"Homelessness today is an economic problem," said Katherine M. Mainzer, executive director of the 1400-member Massachusetts Coalition for the Homeless.

"Whether someone is crazy or sane, people used to be able to rent a room for $20 a week and can no longer do so," Mainzer said. "Not since the Great Depression have the homeless been so evident," she said.

Mainzer said the government does not have a sufficient policy for helping the growing number of transients. "We continue to lurch from one crisis to the next," said Mainzer, calling for greater communication between shelters and the department of mental health.

The third speaker, Philip Johnston, secretary of the state Office of Human Affairs, said he thinks Massachusetts has been effective in aiding the homeless.

"We think we've got a handle on the short-term homeless situation," Johnston said.

But, he added, "It's important not to forget that we're working with a national administration which has turned its back on the poor and homeless," he said.

Since 1983, Johnston said, the number of state-supported homeless shelters has jumped from two to 50. In addition, the state has added a large amount of low-income housing, he said.

Yet skyrocketing rents and cuts in federal social problems make it difficult for the state to alleviate the homeless problem, Johnston said.

Johnston blamed state hospitals for the growing ranks of the mentally ill who are homeless, and praised the administration of Gov. Michael S. Dukakis for trying to reform the state's mental hospital system.

"The more we've looked at the health situation, the more we realize that failures of state mental hospitals have led, in part, to a large number of mentally ill homeless," Johnston said.

Breakey, on the other hand, said, "The notion that large numbers of people are being discharged from hospitals on to streets is not true."