The average American man changes jobs every 4.6 years, the average working woman every 2.8.

That is mobility, a source of national pride.

But the average Amercian pension plan in private industry pays no benefits unless a worker stays with the same employer 10 years.

And that is one of the reasons why the private pension system is of little help to most workers.

Much has been written in recent years about this system.

At $279 billion book value, it is one of the largest private accumulations of capital in history. The great bulk of it is controlled by a few large New York banks; it dominates the stock and bond markets. Vast future pension obligations area a kind of cloud on the books of some of the country's most prosperous corporations.

Yet for all its size and cost, the system so far is producing relatively little in benefits for the 24 million persons over 65.

Only 7 million retired were receiving private pension income in 1975-76 (most were over 65, but some were younger early-retirees). These 7 million received $14.8 billion in benefits in 1975 - an average of $2,204 each.

Even that low average in some respects misleading, moreover, because some private pension plans are elaborate and high-paying, which means that many others pay much less than the norm.

At Chrysler, for example, under the contact negotiated by the United Auto Workers, a worker who has been reached normal retirement age, can Chrysler - financed pension until be becomes eligible for Social Security. At that point his Chrysler pension would drop to about $330 a month, but with his Social Security benefit and his wife's plus the $330, he'd probably end up with about $900to $950 a month.

On the other hand, in a lower-wage, poorer industry plan for the Amalgamated Clothing Workers would give a worker retiring at 65 now after 30 years only $157.50 a month on top of his Social Security.

The result is predictable: a 1973 study by the Bureau of Labor Statistics suggests that private pensions were providing less than 10 percent of the income from investment provided about 20 percent (most elderly people have only a few thousand dollars in cash assets plus the value of their homes). Social Security and other public retirement or welfare payments were the single largest source of income, at around 40 percent. The rest - between a third and a fourth - came from jobs, either part-time or full.

Three -fifths of the nation's elderly persons derive more than half their income from Social Security. And even though Social Security benefits have been raised much faster than the cost of living in recent years, about one elderly person in seven still lived in what the government officially defined as poverty in 1977.

One reason for the paltriness of private pensions is that the systems in relatively new in America.

The system's major growth did not start until 1947, when the government ruled that employers were required to include the subject of pensions within the scope of collective bargaining. That did not mean employers were required to establish private pension plans, only that unions could bring them up in bargaining.

In 1950, only 20 percent of the labor force worked in jobs where a pension system existed, though this did not guarantee them pensions, since many changed jobs too soon to qualify.

This coverage has risen steadily to about 50 percent. The Labor Department and Internal Revenue Service estimate that, as of 1976, some 32.5 million to 35 million workers were in jobs covered by private pension plans or deferred profit-sharing plans similar to pension systems.

It is true that the great majority of these jobs are in the higher - paid basic industries where there are established unions and well-to-do companies.

Nevertheless, the private pension system is spreading. Another sign is this: according to government figures, in 1976 only about 20 percent of married couples and unmarried individuals over 65 were receiving private pensions. But for those currently reaching 65 the number is higher: 32 percent for couples, 18 percent for unmarried.

Despite these encouraging numbers, however, many expects doubt that private pension coverage will ever extend much beyond 60 percent to 65 percent much of the labor force. Moreover, because many workers will never stay long enough in any one place, experts believe that at very best no more than 50 to 60 percent of all those reaching retirement age in the future will actually get pensions. Some put the figure lower - 35 to 40 percent.

A labor Department expert said, "Most of those in large firms are covered. No one knows how much this coverage will grow, if at all. People in small, low-wage struggling businesses are the biggest problem. Candy stores, mom and pop stores, little restaurants. They're unlikely to get into covered jobs or earn pensions. The businesses are too poor."

Paying the tab is another problem. Extend coverage and better benefits mean higher costs, a particularly troublesome prospect in an era when the ratio of retires to younger workers is rising rapidly.

The solution most commonly suggested for curbing pension costs is to induce (or require) workers to retire later, perhaps at 68. The same solution is also suggested for controlling costs of Social Security.

The only other alternatives are forgoing increases or even cutting real benefits or imposing much heavier financial burdens or business and on active workers.

Social Security, enacted in 1935, never was intended to provide a retired person all the income needed to live on after 65. It was intended to provide retirees with about half of what they had earned before retirement. Private pensions and personal savings were expected to supply the other half.

Over the years Social Security has done its job rather well. It covers virtually all workers: it is financed 50-50 by mandatory payroll taxes paid by workers and businesses: and it is administered by the government. The average Social Security benefit today for a couple is $433 a month - pretty good, but not enough to live on for most people unless they have savings and private pensions as well.

Aside from the likelihood that a substantial portion of the labor force will never earn private pensions, the system suffers another serious problem: unlike federal employe pensions and Social Security, the private pensions normally don't provide for automatic cost-of-living increases (only 4 percent do). In an inflationary time, this can be a disaster.

Alicia Munnell, vice president and economist of the Frederal Reserve Bank of Boston, recently told the House Committee on Aging: "Private pensions do not provide benefits which are indexed for inflation. Social Security does. If inflation at 6 percent, the real value of the average 1975 pension plan benefit will be reduced by about in 10 years." (Right now, inflation is up to around 10 percent.)

A major question for government and private bodies who are studying the isssue is how to boost the income of the retired, particularly those at the low-income end who lack private pensions and have littlesavings. A 1969 Social Security study showed that 19 percent of the elderly had no net worth at all; aside from equity in homes, worth about $8,500, half the elderly had net worth under $2,400.

Some like Merton Bernstein, of Washinton University in St. Louis, believe the best way to boost income of the elderly is to stop encouraging the growth of private plans and instead substantially improve Social Security. "Private pensions are not performing their role," he said.

However, in practical political terms, that isn't today a viable alternative. The pension funds have become the single biggest source of private capital in American.

Each year businesses are putting over $30 billion into new reserves to pay off future pension obligations. Correctly or incorrectly, many business and investment counselors believe private pension funds are an absolutely essential way to get money together in big batches to fuel investment and keep the economy going.

"The funds must be available for capital markets," said George Pantos, an attorney whose firm represents the ERISA Industry Committee, made up of officials of big cooperation with pension systems.

Some planners believe the IRA (individual retirement account) authorized by Congress four years ago holds out considerable hope for expansion of pension coverage. Workers who aren't covered by a private pension plan can put up to $1,500 a year in such an account - with a bank of insurance company - and defer paying taxes on both the $1,500 and any interest earned until they reach retirement age and start drawing out the money, at which time their tax rates presumably will be lower.

Already 1.9 million such accounts exist, the Treasury says, and $2 billion went into them in 1977. Undoubtedly many workers in small firms will buy them.

But the lowest-income workers, whose income is greatest, and who are least likely to work in firms with pension systems, can't afford to buy them - and they don't get much of a tax break since their tax rates are lower anyhow. So, IRA's are probably not a solution for them.

In the long run, all proposals - whether for Social Security improvements or more private pension coverage - run up against a massive demographic change whose effects are already being felt - the aging population. Social Security is already under financial strain as a result, and so are private plans, whose unfunded vested pension liabilities are sometimes higher than the net worth of the company (Lookheed's) is nearly double.)

Secretary of Health, Education and Welfare Joseph A. Califane has said the unfunded liabilities of all public and private pension systems may we be over $600 billion.

The population is aging, the elderly are living longer, people are retiring earlier. More than half the persons coming on Social Security now are opting for retirement at age 62 to 64, even with reduced benefits, instead of waiting for 65.

Today, 11 percent of the population is 65 or older: by 2030 it will be 19 percent. Today there are 19 elderly persons for every 100 of normal working age, 20 to 64. By 2030 it will be 34.

That means every active worker will have to pay bigger and bigger taxes to support the elderly. The sharp future rise scheduled in Social Security taxes is one sign of this. How can you improve benefits when society is already straining just to maintain the current level?

Not everyone is clear on how to do it, but there is virtual unanimity from right to left that as the younger labor force shrivels, as people live longer in better health, more of them must continue to work beyond 65. This will reduce overall burdens of Social Secrity and private benefits while boosting income from payroll for Social Sercurity.

"All older people aren't going to work." said former Social Security Commissioner Robert Ball. "That's crazy. But many healthy older people will want to got to work or stay at work.

Slowing the growth of furture benefirs is another money-saving device. One proposal would tighten early - retirement and high - benefit rules for civil service and the military. Military can retire at half pay after 20 years service, regardless of age; federal employes can retire with 56 percent of their highest three-year pay at age 55, after 30 years of service. In 1977, Civil Service Commission costs for retirees under 65 were $2.8 billion. The government actually pays nearly four-fifths of the cost of civil service retirement benefits.

Under current law, it is possible for a person to earn several early - retirement public pensions and Social Security as well, and retire early with a handsome bundle. Former Rep. Hastings Keith (R-Mass), now 62, who left Congress in 1972, criticizes this system and cites his own case an as example: For the past several years he's been receiving a civil service pension based on 14 years in Congress and six in the service which now comes to $2,400 a month. In addition he gets a $600 a month pension for service in the National Guard and reserves, plus $147 from Equitable Life Insurance, for which he worked before going to Congress. And at 62, he's eligible for Social Security as well, though he won't take it.

Although experts say the private pension system probably will always leave out the poorest one-third to one-half of the labor force, this may not be as serious as it sounds.

There is a theory that workers after retirement need about 75 to 80 percent of pre-retirement income. Social Security benefit schedules give the lowest-income workers disproportionately high benefits in relation to their lifetime earnings. The average low-income worker, one who earned perhaps $5,300 in the year before retirement (in today's money), will get Social Security benefits under new schedules equal to about 53 percent of his preretirement earnings. (Higher-income workers get a much lower proportion). If the 50 percent wife's bonus is added, a couple would end up with an income-replaced rate of about 80 percent - tax-free.

Karen Ferguson of the Pension Rights Center, which is holding a conference on these issues next weekend, vehemently disagrees that this is enough. "Even though the replacement ratios may be 65 to 75 percent for the low-income workers, the amounts are too low. People cannot live on these amounts - $3,000, to $4,000 or $5,000 a year.They need more. Sooner or later we must cover these people with something."