In 1972, the number of women applying for jobs at the nation's largest coal producer, Peabody Coal Co. in Kentucky, was zero. But by 1978, following a government affirmative action program to promote coal-mining jobs for women, female applications soared to 1,131.

Before this nationwide Labor Department effort -- which included threats against coal companies for failing to follow guidelines -- 1 of every 10,000 underground miners was female. By 1980, however, 1 of every 12 miners was female.

This dramatic change is among examples cited by the National Academy of Sciences in a study that concludes that women have been effectively shut out of many kinds of jobs and "segregated" into lower-paying occupations largely because of discriminatory barriers, rather than their own choices.

The 173-page report, described as the most comprehensive study of "sex segregation" in the nation's workplaces, criticizes the Reagan administration for decreasing enforcement of affirmative action regulations that the study said have helped to break down those barriers for women.

The two-year survey, sponsored by the Labor and Education departments and the Carnegie Corp., is likely to be used in the debates over goals and timetables for affirmative action and over "pay equity." Pay equity supporters contend that wages for many predominantly female jobs should be raised -- with government intervention, if necessary -- because pay for jobs held mainly by women has been below that for predominantly male jobs requiring similar skills and training.

"We are cognizant our study will be controversial," said Alice S. Ilchman, president of Sarah Lawrence College, who chaired the 14-member committee of academic and business experts that wrote the report for the academy's National Research Council.

The report warns of problems if the administration goes ahead with a proposal to eliminate numerical goals for hiring women and minorities that apply to government contractors under a 1965 executive order. It also criticizes plans by the Equal Employment Opportunity Commission (EEOC), which has said it will move away from class-action lawsuits on employment discrimination and instead pursue more individual cases.

"Our committee found evidence of the significant role that affirmative action enforcement can play," Ilchman said in an interview. "And we did not want to underestimate what government executive leadership can do to change the climate . . . We believe the progress made in recent years has been to a significant extent because of public leadership" in promoting equal opportunity.

The study does not endorse pay equity, also known as comparable worth, but it supports the theory often put forth by pay-equity supporters that hiring and training practices, "social stereotyping" and other roadblocks -- rather than women's preferences -- account for the high concentration of women in lower-paying jobs.

Almost 50 million women were employed in 1984, a record 43 percent of the U.S. labor force. But roughly half of them work in job categories that are at least 80 percent female and that usually have lower pay and benefits than predominantly male job categories. Women, on average, earn 60 percent as much as men, the study said.

The report, drawing on more than 250 academic and government studies of the workplace, said most of the reason for men's higher pay involves higher levels of skill, education and training. But it concluded that "about 35-40 percent of the disparity in average earnings is due to sex segregation" because women are essentially steered into lower-paying jobs by the education system, business personnel practices and other social factors -- including their own beliefs that certain occupations are essentially off-limits to women.

Coal companies previously claimed they couldn't hire women because they never applied for jobs, "and when women do not apply, everyone says it is their own choice dictating that," said Heidi I. Hartmann, an economist who directed the academy's study. "But once it is publicized that women have these opportunities, hordes of women came out of the woodwork and applied. Did their preferences change overnight? . . . Or did they know they would not have been accepted, so didn't bother applying?"

(Researchers noted that the number of female miners has declined sharply since 1980 because of layoffs that disproportionately affect people with low seniority.)

Sex segregation in jobs declined by 10 percent during the 1970s, the report said, a change that coincided with the rise of the feminist movement and an activist enforcement policy by the Labor Department and EEOC.

The report cites the case of American Telephone and Telegraph Co., which in 1970 routinely assigned women to lower-paying jobs and had concentrations of more than 90 percent of one sex in virtually all its jobs. But after government intervention -- including an EEOC challenge to telephone rate increases -- AT&T made dramatic changes in its hiring, promotion and training policies.

But the report said such progress toward integrating workplaces has been slowed in the 1980s. "Decreases in federal enforcement that have occurred since 1981 and recent changes in the philosophy of enforcement, including reversals of federal civil rights policy in some areas, are likely to negatively affect women's future employment opportunities," the report said.

"The reduced effort, or even the perception of it, could affect the behavior of employers," it said.