WHAT CAN you say about a 44-year-old professor who writes bestsellers?

That he likes bad characters, bad plots, and bad endings? It doesn't seem fair to criticize Erich Segal '58 for making a buck the easiest way he knows how. After all, he would probably be the first to admit the slick vapidness of his "novels." Because, of course, they are not novels.

Segal writes screenplays, beautifully thin scripts for beautifully thin people to act in beautifully thin films. When he took his screenplay Love Story to Paramount Pictures more than a decade ago, they told him to turn it into a novel while they were making the film. The ruse worked, and for one glorious week, Love Story stood at the top of both the hardcover and paperback bestseller charts and was the number one grossing movie in the country. Not bad for a Harvard man who teaches Latin and Greek literature at Yale.

Unlike Oliver and Jenny Barrett, who were nearly perfect, Segal has wilted under Mammon's gaze and added Oliver's Story to the screenplay-novel genre. And now comes Man, Woman and Child, the kind of novel a writer produces only thrice in a lifetime, a literary tortilla, a hardcover hors d'oeuvre.

If professor Segal spent more than two weekends writing this modern fairytale, he worked slowly. He probably went to see George Roy Hill's A Little Romance and Robert Benton's Kramer vs. Kramer and decided America wanted more in the cuddly little boy and cute little girl department. With his track record, he may not be far off.

In Man, Woman and Child, Segal obeys all the rules of screenwriting. He writes in short, never-more-than-five-minute scenes in which visual images carry more weight than language. Typically, we hardly know what the characters look like so we won't be disappointed by the movie version.

Hook your audience in the first five minutes, most screenwriting manuals instruct. And there it is, smack on page five, the hook "He is your child too." Poor Bob. He has this perfect marriage with Sheila. They have two lovely daughters, precocious Jessica and wide-eyed Paula. Bob teaches statitics at MIT. Sheila edits books at Harvard University Press. Half way through their twenty year marriage, Bob had an affair with Nicole in the south of France, which he never admitted to Sheila. Nicole is dead but has a son...Bob tells Sheila.

POOR SHEILA. Even though trust has evaporated from their marriage, she still loves Bob too much to leave him; so with a martyr's sense of duty, she agrees to let nine-year-old Jean-Claude visit them for a summer month on the Cape (where else) while French friends try to keep him out of a state orphanage.

Poor Jean-Claude. He just happens to speak perfect English and play soccer expertly. He's so cute that if Justin Henry can learn a French accent he won't be denied the best supporting actor Oscar again.

"When words fail, comfortable cliches are always nice to fall back on," Sheila admits halfway through the book. Segal knows about comfortable cliches all too well. He struggles to add narrative to his script. His prose is not simple or spare--it is empty.

Yet the failure of Man, Woman and Child lies not in its style but its story. Full of dramatic and meaningful scenes that (without giving away and surprises) add up to nothing, they fizzle like a summer squall over Cape Cod. As he proved in the hockey and snow sequences in Love Story, Segal has a sharp eye for powerful movie scenes. Bob & Sheila & Jessica & Paula & Jean-Claude peaks with Bob and Jean-Claude at Arthur Fiedler's fiftieth anniversary July Fourth concert. As they sit in a drenching downpur eating sandwiches, the Old Man throws the Pops into the 1812 Overature as fireworks explode into the night sky. It's sort of like the running sequence in Rocky, only Ivy League.

GOD FORBID SEGAL should ever leave New England or invent characters who didn't go to Harvard, Yale, Radcliffe or Vassar. Or characters who didn't grow up to be professors or lawyers or gallery owners or editors. Or characters who couldn't afford to rent a house for a month at the Cape or eat at Harvest ("the new restaurant behind the Brattle Theater").

Or characters who didn't abandon all virtues of character for a happy ending. Segal's subject--an unexpected child, whosever it is--could make for a good serious novel, one in which family conflict lasted longer than 24 hours. But Segal's real story is still love and his quest still money. He well knows that empty love sells better than profound tragedy. He's already working on another novel. What can you say?