(RNS) Celibacy is thinning the ranks of an already shrinking Catholic priesthood, according to a major study that found that about one in seven newly ordained men resign within the first five years, largely because they feel lonely and unappreciated.

Most new priests are largely satisfied with their work, finding their greatest happiness in celebrating the Eucharist and preaching the gospel. But even some of the most satisfied priests report problems with overwork, said the Catholic University of America study.

For a number of new priests, the problems are overwhelming. The study, led by sociologist Dean Hoge of Catholic University of America in Washington, D.C., estimated that during the 1990s, 10 percent to 15 percent of new priests resigned within five years, up from an estimated 8 percent to 12 percent in the 1980s.

In a survey of 72 priests who left, nearly half said the loneliness of priestly life and celibacy were "great problems." Ninety-four percent said the church should make celibacy optional.

"If they don't want to change the rules, we have to prepare the men for that life," Hoge said in an interview. "The priests have to be prepared for the celibate life better than they are now."

Hoge reported on the study earlier this month at the annual joint meeting of the Religious Research Association and the Society for the Scientific Study of Religion.

The job satisfaction of new priests is critical to the Roman Catholic Church, which has watched its U.S. membership grow to more than 60 million, while ordinations have declined from 771 in 1975 to 442 in 2000.

Hoge said the number of priests ordained in recent years represents only 35 percent of the number needed to replace priests who die, retire or resign from the priesthood.

Researchers last year surveyed 527 active priests randomly selected from 44 dioceses and 44 religious institutes, as well as 72 men who left the priesthood. All were ordained in 1992 or later. The average age of priests at ordination is 36.

Hoge said the margin of error was plus or minus 7 percentage points.

The good news in the findings is that most priests are happy in their work.

Both active priests and those who have resigned said the three most satisfying parts of their work are administering the sacraments, presiding over the liturgy and preaching.

The study also found that more than two-thirds of both groups reported great satisfaction in working with parish laypeople.

The big differences among active and resigned priests were on issues of celibacy and loneliness.

Forty-seven percent of resigned priests but only 8 percent of active priests said living a celibate life was a great problem. Similarly, 46 percent of resigned priests but only 10 percent of active priests said the loneliness of priestly life was a great problem.

Breaking down their interviews with resigned priests, researchers said about a quarter were heterosexual priests who felt lonely and fell in love. Another quarter were priests who were lonely and decided not to continue as celibate.

About 35 percent were priests who felt lonely or unappreciated and were disillusioned with fellow priests or the church hierarchy, and about 10 percent were gay priests who wanted open, long-term relationships. The rest did not fall into one of the four types, Hoge said.

Overall, a major recommendation from both resigned and active priests was to be better prepared for the loneliness and isolation that many feel after the active community life they enjoyed in seminary.

"Seminary training hadn't really prepared them for this type of life," Hoge said. "The priests have to be told, have to be trained in leading this isolated life."

The study was initially proposed by the National Federation of Priests' Councils, which was disturbed by reports that many new clerics were finding the priesthood difficult and discouraging.

Hoge conducted the study as part of a larger Duke University "Pulpit and Pew" project funded by the Lily Endowment.