Psychologists say that filling the role of the responsible firstborn, while important to academic achievement, still does not account for eldest children’s higher average scores on intelligence tests. Robert Zajonc, a psychologist at Stanford University, has argued that in fact having a younger sibling or two diminishes the overall intellectual environment for eldest children  who otherwise would be benefiting from the rich vocabulary and undivided attention of parents.

This helps explain why, under the age of 12, younger siblings actually outshine older ones on I.Q. tests.

Something else is at work, Dr. Zajonc said, and he has found evidence that tutoring  a natural role for older siblings  benefits the teacher more than it does the student. “Explaining something to a younger sibling solidifies your knowledge and allows you to grow more extensively,” he said. “The younger one is asking questions, and challenging meanings and explanations, and that will contribute to the intellectual maturity of the older one.” (Only children receive the benefit of more parental attention but miss the opportunity to tutor a younger brother or sister.)

Ms. Monaco, who has two sons in their 20s, said her oldest was expected to help his brother from an early age. “He was a teacher to his brother, and he has grown up to be a more intense thinker; he’s studying business management,” she said. “His brother is more easygoing, independent; he’s studying leisure and recreation and has an internship at a golf course.” The two are very close friends, she said.

Parents who recognize the different niches that their children fill can enhance the family’s intellectual environment by exploiting each child’s expertise, researchers say. “Given the evidence we have on this, I would as a parent encourage late-born siblings to take on teaching roles, with other siblings or other children,” said Paul Trapnell, a psychologist at the University of Winnipeg.

Dr. Trapnell compared this process to the so-called jigsaw approach used in classrooms, in which complex projects are divided up and each child becomes an expert in a particular task and instructs the others.

Younger siblings often have something more to pass on than the tricks of their favorite hobby, or the philosophy behind their social charm. Evidence suggests that younger siblings are more likely than older ones to take risks based on their knowledge and instincts.