THE PUBLIC TORMENT of Billy Carter, as everyone senses, has passed beyond the familiar context of low comedy and verges now toward something serious, perhaps tragic. No one, at this point, can make him shut his month for his own good. No one, for that matter, can tell the grand jury to get off his back.

I know exactly how he feels. No matter how outrageously cruder or spiteful Billy Carter becomes, or even what legal troubles he may get into, I would still identify with him. When his vile comments appear in print, I would figuratively put an arm around him Okay, Billy they heard you.

Nobody needs to excuse his bad behavior, but it's easy for me to understand it. Billy Carter is the errant little brother. A kid named Billy who never became William (or even Bill). I know every move he makes. He is the last of four strong, smart, articulate children. I know how he feels because I was too.

The last-born child is stock role in the melodrama of large American families, usually a gag character. He is the young wiseacre with a cowlick who says impertinent things at dinner, then walks away smiling as his big brothers and sisters scream at him.

President Carter's family, as he has told us, read books at the dinner table when he was a child. I can imagine little Billy yearning to make a bit of genuine conversation at dinner. Only he is surrounded by all these big people with their faces hidden in books.

So Billy zings them, I imagine. He says something so foul and contentious that they are forced to react. Jimmy probably glowers at him, with the same ice-blue, impatient glare tht he still uses at the White House on fumbling aides.Billy's sisters giggle, indulgently. His Mama or his Papa perhaps scold him, but Billy can tell from their tone of voice that they aren't really deeply angry. Then everyone goes back to their books -- and Billy never gets to say what is on his mind.

Last-born children learn two techniques for dealing with these slights: either to broadcast the pain with weird behavior or to keep your head down and tell no one where you're going after supper. There are a lot of us around, even some in politics, which is not surprising when you think about it. In every large family, somebody has to be last.

Sen. James Abourezk, whose behavior outraged so many of his stuffy colleagues before he retired to make his fortune as a lawyer, is a last-born son. So is Sen. Edward M. Kennedy. The other day I read a perceptive article by Martin Nolan of the Boston Globe, explaining Kennedy's political ambitions in these terms. Kennedy is the last of nine children, but his father and his older brothers are dead. Nothing left to prove. And no one left to prove it to. Nolan understands, because he is one of us.

I AM CONCENTRATING on the boys because I do not pretend to understand how this business of family position works for girls. I assume it's not so different, but I have discovered, like many other American males, that we did not know very much about how our sisters grew up. The same house, the same dinner table, but the experiences were so different.

I know thismuch about the boys -- the last-born feels distant, insulated from contentious emotions, such as the wrath of parents, that provoke so much smoke and flames among the older children. In many ways, he feels uniquely free. He is immune from the stomach knots suffered by his older brothers when their Mama looks crosseyed at them. The family rules still exist, but they no longer are enforced with the same zeal. The last-born are indulged more than punished, especially if they learn to respond to crisis with wit and good humor. If the little brother gets caught, he clowns his way out of it. The injustice of this used to drive my brother wild.

On the other hand, the last-born also feels permanently confused, even chilled, by the unexplained events that occurred long before he came on the scene. He is like a character in a play who enters in the second act and is expected to take up his role, only no one has explained to him what happened in the first act. Family storms seem to come and go in senseless patterns. Everyone else knows why they are shouting, but he's just shouting to keep up with the rest.

Distant -- but free. I always felt there was a kind of rough justice in this tradeoff. I always thought that last was the best place to be in a family, if one could choose when to be born. The bigger people overshadow the new kid and outshout him, but he has a freedom of movement that most of them never will enjoy. Perhaps this feeling of mine merely speaks well for the feelings within my particular family, because Billy Carter certainly proves that the shadows also can be terribly oppressive, confining.

Whatever nasty things you wish to say about him surely Billy Carter has spent more of life in the shadow of his big brother, the president, than ought to be required of any adult. He cannot get away from him, anywhere. Billy ran away to the Marines when he was young, and probably he should have kept going, as far away from Plains as his own talents could take him. But that is easy hindsight now, which hardly helps him.

The errant little brother has two basic choices. He can keep scrambling to catch up with the other characters, hoping to claim eventually a full role in the family script. Or he can throw up his hands and walk out on the story. Make his own story, somewhere else, with other characters. The latter choice will leave twinges of unfinished conversations in his memory, unsettled arguments, but these become less important over time if he finds another interesting script.

I WOULDN'T WANT to spoil this rumination on families by claiming too much relevance for it. But I do believe that what I have been saying about the positions of children tells us something about the way politicians behave, the way we react to them. I can't explain it all very clearly, but then neither can anyone else.

As a rule, America elects the first-born son as president. He is more driven, more uptight, more serious about his own egotism than the rest of us. If he were not, he wouldn't force himself through the humiliating ego bath called campaigning. The eldest son is closest to his parents, more responsive to their signals and, therefore, bound more closely to the orthodox verities of our socity. In the primitive order that still guides us, the oldest son will replace the father -- either concretely as head of the family or psychically in Mana's affection, or both. Therefore, he cares a lot about proving his manhood, proving that he is a worthy successor to Daddy.

If you run down the list of recent presidents, you will see that most of them had errant little brothers like Billy Carter. Edward Nixon, Sam Houston Johnson, Teddy. These presidents came from families where the emotional pressure was turned up high, either by strong parents or by tragic events, illness and death, poverty or financial failure. In a sense, we send big brother to the White House because we trust him -- but then we torment him mercilessly, teasing his seriousness the way kid brothers do.

What makes Edward Kennedy intriguing as a public figure, beyond the royal Irish tragedy of his family legacies, is that Kennedy is both. Teddy, for most of his life, was the errant little brother, a role he played with more flash than wit. Always getting into trouble, always walking away unharmed. Some people hate this in a man, especially people who were older brothers and sisters.

Only now Edeward Kennedy is the eldest male of his family, the responsible one and the last one too. This tension makes him more interesting as a public man than John or Robert were, and also more daring. If he chooses finally between these two roles, that will settle his political future. The eldest son runs for president. But my own guess is that Teddy enjoys too much the freedom of being little brother to give it up gladly.

Of course, there is the option of Jerry Brown, the governor of California, who behaves liek an only child. He is not, but he acts like one. Brown grew up with three sisters, no brothers, the third of four children. We all know what that means.

I think it would be interesting someday to elect an errant little brother as president, but America isn't ready. I am not certain that the last-born son would be any less dangerous than his big brother. An errant little brother in the Oval Office might punch the button on his desk some morning, just to see if it brings the butler or World War III.

The creative solution, obviously, is for the republic to elect an oldest daughter. She will be driven and uptight too, but at least she will display some originality of style. Especially when she must demonstrate as president that, just like Daddy, she is a real man.