An Allegory of One Child Too Many : THE FIFTH CHILD <i> by Doris Lessing (Alfred A. Knopf: $16.95; 131 pp.) </i>

For all our time’s troubles, and presentiments of more to come, its complexion still shines reasonably rosy. Doris Lessing anatomizes the rosiness as a matter of blood congesting in weakened capillaries that will not hold much longer.

“The Fifth Child” is a gaunt fable that begins as a cheerful story of a fond if fatuous marriage, assumes a guise of near-Gothic horror and finally strips to disclose a desolate social message.

It is a brief work, undeniably contrived and surprisingly lethal. Lessing shifts her targets surreptitiously. We think we are accompanying the author on a comfortably satiric duckhunt. The ducks in question are the self-congratulatory David and Harriet Lovatt, with their insistence on establishing a big, sloppy, five-child family in an age of careful one-child self-actualization, and enjoying the appalled plaudits of “We Don’t See How You Can Do It” and “Aren’t You Wonderful.”

Lessing wings them, all right; suddenly, we see the torn flesh and staggering flight. And by the end, the gun quietly points at us.

We meet Harriet and David as they are about to meet each other at a 1960s London party. Neither one approves of the party nor of the ‘60s either. They are old-fashioned non-swingers by conviction and, Lessing tells us, they are drawn to each other by their “mutual air of discomfort.”

The author is breezily high-handed in setting the stage for a deliberately stagey production. She places her characters just where she wants us to see them. In suggesting Harriet’s counter-current stroke, she writes that she regarded her virginity as “something like a present wrapped up in layers of deliciously pretty paper, to be given, with discretion, to the right person.”

David, home-loving son of a broken family, is the right person. They buy a big, unfashionable house in the London suburbs and, long before it is sensible, Harriet is pregnant. David’s trendily intellectual mother is concerned; how will they afford it? David’s wealthy father pitches in. Harriet’s kind, domestically minded mother is concerned; how will they cope with the work? She pitches in.

Three more children follow in quick succession; with more pitching in, financial and manual, respectively, by David’s father and Harriet’s mother. Numbers attract numbers; before long, the Lovatt household is the regular venue for Easter and Christmas reunions at which two dozen assorted relatives, admiring and with large appetites, come and stay for weeks. The admiration persists, but the concern gradually turns to alarm.

It is comic satire so far. But Harriet’s fifth pregnancy shifts the tone. It is a violent pregnancy; the unborn child is never still. It becomes a near-war; the fetus kicks savagely and continually, the mother retaliates with tranquilizers. And when Ben is finally born, he is coarse-haired, sallow and somewhere on the verge of monstrosity.

The monstrosity grows. Nursing, he savages Harriet’s nipple. Put on a bottle, he consumes two at a time. A few months old, he sprains his brother’s arm yanking him into the crib. At 1 year old, he strangles a puppy. He is an interloper, almost a different species, and increasingly uncontrollable.

Distraught and exhausted, the parents agree to their relatives’ scheme to put Ben in an institution. But when Harriet goes to visit, it is a place of nightmare, where retarded or deformed children are kept in strait jackets, drugged and left to die.

She brings Ben home, to the despair of David and the other children. Ben grows, brutish, sullen and violent, and the family falls apart. The three older children go off to boarding school to get away from him and spend their vacations with other relatives. David stays, working to support them, but almost totally estranged.

There are other horrors, and we expect a ghastly climax. In fact, the climax has taken place. It is the rupture of the Lovatts’ family fantasy of the good, generous and abundant life by this seeming intruder. A kind of miserable truce sets in, as Ben grows into adolescence. He joins a local gang, which alternately camps out in the house, and vanishes for weeks at a time.

The active horror has departed, in fact, from the Lovatts’ cold and shattered heart. And gradually, we see what Lessing is up to.

Ben, the monster, has become almost normal. Normal, that is, if you consider the hordes of unemployed, unattended, marginally criminal youths that Britain’s social and economic plans have no place for. Ben is not an invader from outer space but from the lower-class. The contagion in the Lovatts’ old-fashioned paradise is not biological but social.

Lessing’s fable, simplistic and overmaneuvered as it sometimes is, achieves not only power but breadth. The monstrous Ben represents an underclass too numerous for the closets to contain. He is the Third and Fourth worlds, casting a growing and darkening shadow over the comforts of the First.

At the end, Harriet--no longer a figure of fun but our own tragic precursor--sits in her empty house and wonders how long it will be before Ben disappears for good into a slum or a jail. Or maybe even abroad.

“Perhaps,” she reflects, “she would be looking at the box, and there, in a shot on the news of Berlin, Madrid, Los Angeles, Buenos Aires, she would see Ben, standing rather apart from the crowd, staring at the camera with his goblin eyes, or searching the faces in the crowd for another of his own kind.”