In his State of the Union address, President Reagan boasted that during his administration more Americans held jobs than at any other time in history.

But at the same time, more people are now unemployed than when he took office, and the unemployment rate is not expected to decline significantly through the end of the decade.

The unemployment rate has risen in two consecutive months, from 7.1 percent in November to 7.4 percent last month. When Reagan took office, the jobless rate was 7.5 percent. Many economists expect the rate to decline to an average of only about 7 percent by the end of the decade.

The consequences of such extended high joblessness are unclear, economists say, but some studies predict social and economic damage, with more illness, higher payments for unemployment benefits, increased crime and, ultimately, lower productivity because unemployment will cause many workers to be undertrained.

The president's budget and the Council of Economic Advisers economic report issued early this month made little mention of unemployment, what should be done about it or what effect chronically high rates of unemployment will have on the economy. The administration maintained that its major economic goal was to keep inflation down.

According to administration projections, the civilian unemployment rate by 1988 -- the end of the president's second term -- will average 6.4 percent. Projections by the Congressional Budget Office put that figure at 6.6 percent, and private economists such as Data Resources Inc. project a 7.2 percent rate in 1987, as far out as their projections go.

Moreover, last month there were 5.3 million people working part-time because they could not find full-time jobs or because work was slack, according to a new series of statistics by the Bureau of Labor Statistics.

Another Labor Department measure of unemployment that takes into account discouraged workers and those who work part-time for economic reasons shows the unemployment rate for the fourth quarter of last year would be 10.8 percent instead of the reported 7.2 percent.

In the 1960s, government officials aimed for an unemployment rate of about 4 percent, the lowest that rate could drop before inflation would accelerate. Now, that "natural" rate is considered to be about 6 percent.

Part of the reason for the rise in the natural rate is that the labor force has expanded faster than the rate at which jobs are created because of the influx of women, teen-agers and minorities, economists said.

Additionally, some economists said the high unemployment rate is caused by fundamental changes such as the loss of assembly-line jobs to robots.

Roger Brinner, chief economist for Data Resources Inc., is among those who believe long-term high unemployment can spawn social problems. "People have shown that the crime rates are positively correlated with employment, health is affected because if you have a weaker economy people have less to spend on health," he said. "You certainly, from a social point of view, pay a price from higher unemployment rates."

Martin Bailey, an economist at the Brookings Institution, said that the victims of chronically high unemployment don't get the job experience and good work habits that job holders have, making it still more difficult for them to find employment. This problem is particularly acute with young people, who generally have higher than normal unemployment rates.

The lack of experience reduces productivity "and the ability to generate high employment in the future," Bailey said. This then raises the natural rate of unemployment over time.

"I wouldn't think one would observe big general effects on the economy" resulting from unemployment rates of about 6 percent, said Marvin Kosters, an economist with the American Enterprise Institute. "I would think what would be more of a concern is the implications for individuals or groups or areas affected by unemployment."

A Johns Hopkins University professor recently linked the sharp rise in unemployment during the 1973-74 recession to an increase of nearly 46,000 deaths from all causes, including a 2.8 percent rise in fatalities from heart attacks. Mental hospital admissions rose 6 percent beyond the normal trend while arrests increased 6 percent, reported assaults rose 1.1 percent and 1 percent more suicides were reported, the professor said.

The 7 percent of the population who will be unemployed for the next year or so does not consist entirely of long-term unemployed; many work for several months and then collect unemployment insurance for several months before finding another job, economists said.

According to recent Labor Department statistics, managerial and professional workers have less of a tendency to be unemployed for long periods than laborers, repair people, machine operators and craft workers.

"The young, blacks, unskilled will bear the brunt of unemployment" through the end of the decade, Bailey said.

In some areas of the country, such as those with large concentrations of smokestack industries, "there probably is a sense that it's going to be difficult . . . to transform themselves quickly," Kosters said. "That notion suggests that unemployment is difficult to bring down as quickly as one might be able to if there were a completely balanced expansion. My own feeling is there has been a more significant dislocation occurring in the economy in recent years" than previously.

"There are jobs being created, and they would have a tendency to absorb these people. However, the jobs may not be created in Wheeling, W. Va., or wherever," Kosters said.

"Independent of geographic location, even if new jobs are being created it's not so much a question of transforming some individual from his more or less pedestrian accounting practices of an earlier age to a computer programmer. . . . The person whose task has become outmoded has to find himself another task. Anything on the horizon suggesting restructuring of the production horizon . . . means there will be some frictional unemployment.

"Some people will basically never get on a good job path again," Kosters said. "There will always be some who became good at a narrow job category and become too disillusioned at losing a job and never get a hold again."