The tax-deferred investment plan Uncle Sam is offering his employes is, according to private fringe benefit experts, one of the best in the nation. But so far, only about one-third of the people who have the most to gain from the program, known as the Thrift Savings Plan (TSP), have signed up.

The generous TSP program is included in a new pension plan called the Federal Employees Retirement System (FERS), which has been offered to government workers. The FERS plan is specially designed to provide portable benefits for the large number (one of every three current employes) who will not make federal service a career. Yet less than 1 percent of those eligible to sign up for FERS have done so.

Granted, major career and/or financial decisions are involved. But similar offerings in a private company where the average salary was $27,000, the current federal government average, would probably have many more takers. So why are federal employes holding back? Is it the alleged natural caution of the bureaucrat, or something deeper?

The people with the answers are government workers themselves. What many say amounts to this: They simply do not trust the company they work for, the U.S. government. Any benefit offered by the bosses, cynics say, must have a catch. Or if it is good, it won't last.

In talks with many federal workers, that theme -- that they don't trust the politicians -- comes through again and again. One of the best examples of that school of thought came in a letter (condensed here) to the Monday Morning Quarterback section. This is what it said: "I would like to offer a point of view, based on my very unscientific polling, as to why federal workers are slow to jump into any new program, or take any benefit offered by Congress. The point is simply this: We don't trust them. 'We' being federal government employes and 'them' being politicians.

"I cannot recall, nor would you have the space to list, all the broken promises we have had from Congress and the White House, whether Democrat or Republican. Individual Retirement Accounts were authorized by legislation to encourage all of us to provide for our old age. Along comes a later Congress and eliminates the program, as part of tax reform, for most middle-income people.

"Federal retirees at one time got cost of living adjustment (COLA) raises whenever inflation reached, and held at, a certain level. Congress changed that . . . to twice-yearly COLAs. Then to one COLA per year. Then to sometimes-COLAs. What next? No COLAs.

"Federal employes were promised raises to keep pay equal to private industry. What we got is a series . . . of half measures (or pay freezes) dictated by political reasons rather {than} . . . equity.

"We were handed a new pension plan, FERS. One of its benefits was that retirees could take lump sum tax-free withdrawals of their own pension plan contributions. Within months, Congress repealed that option. FERS also promised better Social Security for federal workers who are also either the survivors or spouses of people covered by Social Security. Now the House of Representatives is poised to remove that benefit.

"I went to a meeting of over 200 mid- and senior-level employes on the new retirement system. After an hour of extolling its virtues (and it has many), the 'facilitator' asked for a show of hands as to how many planned to join the new system. Not one hand was raised. Why not? I think I know. We are not timid, and we are not stupid. We are, however, suspicious. And for good reason. The track record is there and the track is muddy." N.P.