THE REAGAN administration has launched a radical economic experiment based ontop officials' Promethean understanding of the psyches of American workers, among whom they have spent so much of their lives.

With almost preternatural insight, the new economic team has divined that only one question burns in the mind of the modern American worker: What is my marginal tax rate?

Let us look in on a typical American family as they make crucial economic decisions on a typical workaday morning in the post-Keynesian era:

The digital alarm woke John Glat Workingstiff at 7:14 a.m., confronting him with the first of many rational choices of the day. What would be the marginal utility of 10 minutes of additional sleep? Could he increase his family's economic well-being more by resting, thus boosting his productivity later in the day, or by rising early to check the economic indicators? Already the London gold market had been open for three hours.Wearily, Workingstiff ross to his feet.

"Orange, pineapple or grapefruit juice this morning, dear?" asked his long-suffering wife, Marge

"It's all fungible to me," he said, grateful that the citrus-supply curves were for once in equilibrium. He slumped at the table, unkempt, the picture of stagnation.

"Is something wrong, dear?" Marge asked. "You seem a little recessed this morning."

"I don't know what it is. My productivity's been slipping at work. I was up all night with nagging inflationary expectations. And I've been troubled by incessant wage demands."

She frowned and felt his forehead. "Feels like a touch of English disease," she said. "There's a lot of it going around."

"Where are the deductions?" he asked.

Kemp, 10, and Roth, 8, were at their Radio Shack microcomputer consoles. "We're going short in rutabagas today, Dad," said the eldest, his shoebutton eyes alight with glee. "We think we can makde $1,000 by the closing bell."

"Before or after taxes?"

Kemp's face fell, "Aw, give us a break, Dad," he said. "It's unearned incomed."

Workingstiff was stern. "You know the rules," he said. "If you can't net enough income today to offset my property tax, you have to recoup it by going to school."

"Aw, Dad, inflation is killing us," the two boys cried in unison. "How about a little indexing?"

Meanwhile, back in the kitchen, Marge was serving yet another plate of charred eggs.He looked at her in her Taiwanese housecoat, her hair in curlers, store-brand face cream on forehead, and though wistfully of the $3,468.43 he had paid in marriage tax last year.

He closed the kitchen so the children wouldn't hear.

"Marge, 'i think the viability is gone from our partnership," he said. "I've been talking with our accountant. He suggested that we check the tax benefits of a Haitian divorce."

"There's a fallacy in your assumption, dear," she replied sweetly. "Let me show you some figures from the Bureau of Labor Statistics about the worth in 1967 constant dollars of my services in the home -- food preparation, child care, domestic cleaning and household supply barter."

From his memory emerged an image of her last summer in her white bikini at "Interest Deduction," their second home on the lake. "Sorry, dear, you're right. I forgot to factor in the seasonal adjustments."

Workingstiff glanced at the digital clock. 8:36. His shift at the Wonder Widget factory was set to start at 9. But what did it matter if he was a little late? Vernal union officials would protect his featherbedding habits. And his union dues were fully deductible.

Anyway, why shouldn't he stay home and miss a half day's work? The worst that could happen was that he would lose his job. Unemployment benefits, after all, were tax free. And he'd probably get trade adjustment assistance and food stamps as well.

But even with this acute attack of English disease, Workingstiff remained a rational man. He decided to calculate the cost of alternate means of transporation to the plant. "Hey, kids," he called into the living room, "get the news service on your screen and check the price of oil on the Rotterdam spot market."

Suddenly, burbles of excitement echoed from Kemp and Roth. "Daddy!" they cried. "In a dramatic late-night session, Congress passed a major tax-cut bill! It's retroactive! President Reagan is to sign it this morning!"

But Workingstiff was apathetic. "I don't care about a general tax bill," he grumbled. "What does it do to my marginal tax rate?"

"That's cut 10 percent in the first year!" said Kemp.

"And there's more to come. And it's predictible!" chimed in Roth.

The effect,though not Galbraithian, was galvanic. Workingstiff reached for his coat, calculator and lunch pail. "Marge, rev the Toyota."

As he swung behind the wheel, he felt bullish on American. He vowed to use his first tax savings to unload the Corolla for a fuel-efficient, but comfortable, Chrysler K-Car. "Don't wait dinner for me, honey," he shouted to Marge, "I think I'll work the second shift."

He backed down the drive and into Elm Street. Around him spread the productive pageant of his beloved Anytown, U.S.A., bustling with economic vigor and unleashed productive energy. Before him, in the direction of the Wonder Widget factory, loomed high vistas of achievement: zooming productivity, tranquil labor-managment relations, victory over the Japanese and Germans. As his father had before him in 194l, he turned toward those mountains, and his face was bright with hope.