A majority of American presidents were first-born children. A majority of presidential assassins were younger brothers.

Jimmy Carter was an oldest child. So were John Glenn and most of the other original astronauts. And something like 40 percent of the people in Who's Who in America and on Time's cover are either first-borns or only children.

On the other hand, Ronald Reagan and Edward M. Kennedy were last-borns.

So was Billy Carter. And most assassins of presidents were younger brothers.

Being first- or last-born does not guarantee success or failure, happiness or misery. But birth order and sibling position usually "tell us important things," a world authority on the subject said here last week.

"Sibling position" includes such matters as whether the olders and youngers were boys or girls, the total number of children and the number of years that separate them.

"In four cases in five," said Dr. Walter Toman of the University of Erlangen-Nurenberg in Germany, "when we have this information, we can make certain anticipations which turn out to be true."

These "anticipations" were explored last week at a conference held by the Georgetown University Family Center, one of many treatment centers using such data to help patients.

Such use in psychotherapy is growing, said Toman, "because once you understand and confront your own fate and the conflicts you had with your parents and peers, you can go on to maturity. We can't help our fate. But within that given, we can accept or not. We can grow or stay arrested."

Remember, said Toman, "none of these trends are inevitable" and there are many exceptions, "though usually there is a reason for the exceptions." But, given these variations and shadowings, here are some of the trends. The words in quotation marks are Toman's.

The first-borns. First-borns and older siblings in general "learn to identify with their parents and teachers and others they perceive as leaders.Because they're first, you see, their first major task is to lead others -- to take care of their younger brothers and sisters." Throughout life, they remain "first" and leaders.

Older persons of achievement -- so far, mainly males, though women are gaining -- have another characteristic. They come predominantly from families of mainly boys. "This seems to make them even more competitive, even more vying for position, for power, for excellence in outdoing the others."

First-borns, of course, may also have problems. The greater demands on them, or the demands or failures they imagine, often drive them into psychotherapists' hands.

Why are only children often achievers when they have no one in the family to compete with?

They, too, grow up identifying with parents and seeking only their attention. Then they get to school. "And there, too, there is a fight for position and power among peers. The only child remains strangely aloof from that. He vies only for the teacher's attention, and this makes him peculiarly attractive to the teacher and schoolmates." Stamped as an achiever, he is thus rewarded and the pattern reinforces itself.

The middle child? "Yes, what you hear so often is true. He or she often has a hard time.His identity may be hard to establish. He has difficulty in finding his position.

"Somebody else is always the oldest, you see, somebody is always the youngest. And he was youngest for a while, then dethroned."

But the middle children, in reply, often become highly adept at handling others, older or younger.

The youngest? "They're typically more carefree. They reply more and lean more on others. That's what they learned.

"But they also may oppose or compete with their older siblings. At first they depended on them, but after a while they may want to outdo them."

Or, sometimes, they choose a different course, sports, say, rather than studies.

"They often relate less to their parents, because they came later and had others to relate to. They care less about what their parents think. They have less motivation to do what their teachers want. So they sometimes appear less intelligent because their marks are poorer."

Some scientists think real intelligence truly tends to decline, on the average, with each successive child -- though, of course, there are differences and exceptions within families.

Toman does not think the intelligence difference is real. He thinks intelligence test have actually measured motivation, which indeed may be less in younger children.

Alos, says another psychologists, the more older brothers or sisters a child has, the more his apparent "intellectual level" is diluted simply by more contact with young children rather than parents.

Which children do best in marriage and other relationships with the opposite sex? Those who grow up with both brothers and sisters, or at least with siblings of opposite sex.

"Only children tend to be slightly deficient here. They haven't learned to live with their peers.

"Oldest or youngest brothers of families of boys, and oldest or youngest sisters of families of girls, also tend to have higher divorce rates.

"Especially if they marry each other. They're not used to living with a peer of the opposite sex."

The "poorest match" of all, other things being equal, may be ignited when "an oldest brother marries an oldest sister and both struggle for dominance, and neither will budge."

The best match? One with a "zero divorce rate," as found in a study of 2,300 German and Swiss divorce (in countries where divorces are still few, though increasing). "When an older brother of a sister or sisters marries a younger sister of brothers. Or a younger brother of sisters marries an older sister of brothers.

"These boys have lived with a girl, and the girls with a boy. So they can live with a man or a woman."

What if a person recognizes some of the less favorable trends in him or herself?

Toman repeated: "We can understand our conflicts and go on from there or we can submit to them. And it all depends on how we handle them."

"For instance, an older sister of a brother may be a motherly, loving, protective person. Or she may become overprotective, anxious and power-hungry, always trying to subdue 'the little boy.' That wouldn't do him or her much good.

"So, you see, there are various ways of maturing."

Many other elements of course affect families and individuals, and particularly so today, said Toman, whether or not a family stays intact.

He mourns the continuing decline of the intact family, the increasing number of children born illegitimately and, he fears, a growing number of children who may be born because a woman is denied an abortion, "as sad as it is to have an abortion."

In all these cases, he said, the children are unwanted by one or both parents, "and I am afraid we are certain to see increasing rates of youth crime, drug-taking and seemingly senseless violence. And neuroses and psychoses and, saddest, future parents who can offer their own children very little because they were offered little."

Answers? "Single female parents must seek father-substitutes, whether sexual partners or not. Or at least mutual circles of women who support each other, and give children support from someone who isn't the parent."

But "this is only one answer that individuals and institutions and governments must develop" if we are to have anything like "the families that make healthy individuals."