Somehow Irwin Shaw, a novelist of the old school, hasn't figured out the new mood sweeping America, the emerging theme of the 1980s reflected in the Reagan tax bill, the hit movie "Arthur" and last month's obsession with the royal wedding. Forget thrift shops, backpacks and Hamburger Helper. This is the decade when it's again fun to be rich.

"Bread Upon the Waters," Shaw's 11th novel, seems primarily motivated by liberal guilt. Everything in this modest, carefully crafted book is Book World BREAD UPON THE WATERS. By Irwin Shaw. Delacorte. 438 pp. $14.95 designed to buttress a very old-fashioned moral point: Money can't buy happiness. Every scene, every character, every turn of the plot is harnessed to the wagon of Shaw's sermon. The resulting novel is mechanistic, predictable and didactic, but, it also is often affecting.

Shaw constructs a sociological experiment around the Strands -- a rarity in both life and modern American fiction -- an intact, seemingly happy, middle-class family living on the Upper West Side of Manhattan. It's Marjorie Morningstar revisited, except the Strands aren't even Jewish. But the symbols that characterize their lives are all lifted from the 1940s. There is the rent-controlled apartment, the friendly doorman, the piano in the living room, tennis in Central Park, education at City College and Sunday trips to the Museum of Modern Art.

The father, Allen Strand, 50, still musters some idealism about teaching history in an inner-city high school. His wife, Leslie, doesn't really work; instead she supplements the family's limited income by giving music lessons in their apartment. There are two children in their late teens living at home, Jimmy and Caroline. The oldest child, Eleanor, who in the 1940s would have been described as a career girl, has her own small apartment, but she still dutifully comes home for dinner on Fridays. As one of the children puts it, "We're rich in affection around here, but when it comes to worldly goods . . ."

This placid family scene is disrupted by the arrival of Russell Hazen, a millionaire Wall Street lawyer who might have been stolen from a novel by Louis Auchincloss. Hazen, blood gushing from a wound on his scalp, stumbles into the Strand apartment after daughter Caroline has rescued him from three muggers in Central Park by swatting each of them with her Head tennis racket. This unlikely encounter is the catalyst that sets Shaw's sociological experiment into motion.

Hazen is the obverse of Allen Strand. Rich where Strand is poor, cynical where Strand is idealistic, active where Strand is contemplative. He too had a wife, a son and two daughters. But his estranged wife is a termagant, his son overdosed on heroin and the less said about the two daughters the better.

Lonely and entranced with their warm family life, Hazen becomes a one-man Ford Foundation for the Strands. It starts with little things -- a new Head tennis racket for Caroline, free tickets to the Philharmonic, a weekend in the Hamptons. Before long, however, Hazen gives each member of the family an opportunity to live out secret fantasies. For Caroline, it is an chance to go to college away from the grimy streets of Manhattan. For 19-year-old Jimmy, it is a first job in the record business. For Allen, it is a prep school scholarship for his brightest student, a Puerto Rican near-delinquent named Jesus Romero.

This is not Goethe; the Strands do not mortgage their souls to the devil. But each time the Strands yield to temptation, there is another fissure in the edifice of the family. It isn't greed that undermines the Strands, so much as passivity. As Hazen displaces Allen Strand as the family patriarch, the family loses its rudder, its inner sense of direction and purpose.

What Shaw is concerned about is the erosion of liberal values. For a generation, the Strands resisted the blandishments of the flash-and-trash materialism of our age. They maintained the veneer of a happy family because their aspirations matched their resources. But when Hazen enters their lives, they become as covetous as any contestant on a television game show. Tempted by Hazen's altruism, the family quickly loses the values that had long sustained them. It is welfare dependency all over again, except the Strands are wards of Hazen instead of the federal government.

This becomes clear as Allen Strand, convalescing from a heart attack, moves from his beloved New York apartment to the dreary faculty housing of a Connecticut prep school. The new job, a virtual sinecure, is, of course, the work of Hazen. Joining Strand on campus is the strong-willed and self-destructive scholarship student, Jesus Romero. Having spent his whole life on welfare, Romero understands the dark side of altruism. Unlike the Strands, he refuses to extinguish his own identity in order to adapt to Hazen's world.

The grim inevitability of the Strand family's decay can strain credulity. In trying to convey a moral message, Shaw sacrifices subtlety and character development for intellectual consistency. But there is a haunting quality to the novel that transcends the sometimes stilted dialogue and the occasional didactic passages, and has remained with me even weeks later.

This ineradicable mood flows directly from the liberal values that Shaw so openly preaches. We can appreciate the corrosive effects of having too much money. Like Hazen, we hunger for the soothing rhythms of traditional family life. We even admire Allen Strand's quiet faith that teaching in an inner-city high school is a higher moral calling than gracing the faculty of an isolated prep school.

Shaw left me with a small realization: Conservatives may be running the government, but bleeding-heart liberals still write better novels.