There are genetic differences, of course, but what explains the very different personalities of children born of the same parents into essentially the same environment?

It is not first off, the same environment: Each child, depending upon his birth-order position, is born into an environment that is entirely different from his siblings'.

Much investigation has been done on ways in which birth order influences a child's development. The person generally credited as the father of birth-order theory is Dr. Alfred Adler, an early associate of Sigmund Freud. Psychologist Lucille Forer, co-author of "The Birth Order Factor," is among those who have recently examined effects of birth order.

What, according to her and others, are generally shared characteristics of the first, middle, last, or only child.

At first glance, it would seem as if the firstborn in a family has an enviable position. He/she is usually welcomed with great joy and lavished with attention. Firstborns, however, are the most highly represented group of children seeking psychological help. What accounts for this paradox?

To begin with, most parents have great expectations for their firstborns, expectations that the children may never be able to live up to. Moreover, the center of attention does not last forever: Mother and Father bring home a second baby and the firstborn's life is never quite the same again.

To compensate for this loss of being the sole recipient of the parents' love, firstborns tend toward overachievement, especially in intellectual pursuits. Firstborns fill the pages of Who's Who; of our American presidents, half have been either firstborns or first-born sons.

Only children, too, are likely to be achievement-oriented. But unlike the firstborn, who is followed eventually by other children, the only child keeps the center stage. Since the only child doen'st have to share his parents with other siblings, the need to shine may not be as consuming.

Often the first child must adhere to a rigid set of rules. This is due partly to the parents' inexperience and anxiousness to do everything "right." This applies to the only child as well, although the firstborn has the added burden of being the eldest, therefore expected to set an example for younger siblings. As a result, firstborns and "onlies" tend in later life to be conservative and place great emphasis on rules and regulations.

Firstborns also tend to "lay down the law" to their younger siblings, a trait not always appreciated.

Research conducted by psychologists Robert Zajonc and Gregory Marcus of the University of Michigan in Ann Arbor indicates that through teaching younger brothers and sisters, an older child reinforces his own learning, which may explain in part why firstborn children are more likely to be better students than are later-born.

Since second children often become middle children later, similarities between these two positions are striking.

On the positive side, later-born children tend to be more friendly and less demanding than firstborns. This is especially true of the middle child who -- because of his experience in dealing with an older, more powerful sibling and a demanding younger one -- develops extraordinary skills in handling others.

Second and middle-born children also share the advantage of having parents who are more relaxed. Later-born children are more likely to look for assistance from others, since they are in the habit of turning to older siblings for protection.

On the negative side is constant competition with older siblings. Usually the firstborn is a few steps ahead in strength and intellectual development, and the later-born child races to catch up. The competition is even more fierce when the older child is of the same sex. Probably as a mean of defining himself, the second child will choose a path very different from that of the first-born. For example, if the firstborn is a scholar, the second may choose sports.

Added to this is the second child's need not to conform -- a contrast to his conservative older sibling.

In some ways the last-born position can be likened to that of the only child in that no further children come along to displace him. But what this can mean is that no matter how chronologically old, intellectually mature or physically developed, the youngest will never have to relinquish his role as the family's baby. And parents are frequently a little reluctant to allow their "baby" to grow up.

But being the youngest has its advantages. The last-born is often showered with affection, by both his parents and older siblings.

As a result, the youngest is often the most charming, happy, fun-loving (some say spiritual) member of the family. And because he has so many teachers and role models, he is likely to learn to walk, talk, and read earlier than older brothers and sisters.

At the same time, the youngest is driven by the need to keep up with, or even do better than, his older siblings. However, according to Dr. Adler, if the older siblings present too much of a challenge, the youngest may lose confidence. He may still be ambitious, but avoid real tests of his abilities and try to wriggle out of difficult situations.

There are, of course, many variables. Sex is one. Is the firstborn a boy followed by a sister, a girl followed by a brother, or are both children of the same sex?

In the older sister-younger sister combination, for example, the firstborn is often a high achiever; the younger sister more competitive, even defiant. The younger sister is more likely to marry and have children before her older sister.

"The center birth position," says Dr. Forer, "seems to affect girls more strongly than boys."

The most difficult position seems to be the middle of three girls. The easiest middle position is that of the middle child of the opposite sex. This gives the middle child a certain status and immediately distinguishes him or her.

Another important variable is spacing. If there are five or more years between two of the middle children in a large family, that gap may generate a splitting into two groups within the family -- the result of a natural identification with those closest in age -- which may change the whole birth-order picture.

Recent research -- such as that by the University of Michigan's Zajonc and Marcus -- suggests that three to four years may be ideal spacing.

Today's parents' tendency to make choices about size of their families may alter the influence of birth order. One of the great advantages of the larger family, for instance, is that sibling rivalry is not as intense. It is possible, psychologists say, that the trend toward smaller families could result in a more competitive society.

Because of each child's uniqueness, psychologists advise -- even if it were possible -- not treating them the same. Psychologist Fitzhugh Dodson in his book, "How to Parent," emphasizes an individualized approach, depending on the child's temperament and position in the family.

Dr. William Homan, in "Child Sense," suggests giving the firstborn special compensation for the stresses placed upon him: for example, a privilege such as being allowed to stay up 15 or so minutes later than younger brothers and sisters. What he does not need, Homan says, is "to be pushed toward independence (and) to be told that he is a big boy and should act his age."

In the two-child or larger family, each needs to feel achievement in some area. Above all, say psychologists, try not to compare one to the other. ("Johnny, look at the way your brother keeps his room" will not be a motivating force for sloppy John.) And be wary of the older dominating and teasing the younger.

In broad strokes, these are some of the special needs of each position that a parent might keep in mind:

The oldest needs less pressure and special attention when the new baby arrives; the middle child needs help in defining his place in the family; the youngest needs assistance in gaining independence.

Remember, too, that no birth-order position is intrinsically better than another. In "The Birth Order Factor," Forer relates the experiences of six people, each of whom had a different position in their families and each of whom felt that his or her position was the best. 1st, 2nd, 5th, 9th

This year's presidential candidates:

Jimmy Carter: The oldest of foour.

Ronald Reagan: The younger of two.

John Anderson; The fifth of six.

Edward Kennedy: The youngest of nine.