The new evidence that eldest children develop higher I.Q.’s than their siblings has intensified the debate over two of the most stubborn questions in social science: What are the family dynamics that enhance intelligence? And can they — and should they — be changed?

Dr. Frank Sulloway, an expert on family dynamics, answered readers' questions about birth order and intelligence. Read his answers. 

The new findings, from a landmark study published Friday, showed that eldest children had a slight but significant edge in I.Q. — an average of three points over the closest sibling. And it found that the difference was not because of biological factors but the psychological interplay of parents and children.

Predictably, the study set off a swarm of Internet commentary from parents, social scientists and others, speculating about what in families could enrich one child’s intellectual environment more than others’.

“Anyone with siblings wonders about this,” said Sue Monaco, 51, of Delaware, who has two sons and five siblings. She was one of about 150 readers who posted questions on Friday to a New York Times Web forum about the study.

Researchers acknowledge that few of the family variables affecting intelligence are well understood, and some argue that peer influences are eventually more significant. But studies suggest that two elements are important during childhood: the perceived role a child has in the family; and the apparent benefit a child receives when he or she tutors someone else, like a younger sibling.

Well before entering the high school hothouse of geeks and jocks, children who grow up with siblings get tagged with labels: The screw-up of the family. The airhead, the klutz, the whiner. And then there is the serious one, little Mr. or Ms. Responsible, who most often is the eldest, psychologists have found.

“In our family we had the straight one, the oldest, followed by the one who snuck out,” said Elisabeth Ferris, 55, a former teacher who lives near Baltimore. “I was the one who snuck out, who had a lot more fun in high school, and who went to art school.”

Studies suggest that other family members tend to consider the eldest the most conscientious of the siblings, more likely to achieve academically. At least for some firstborns, that role may be self-fulfilling.

“I don’t know about our I.Q.’s but, yes, she was the more studious one,” Ms. Ferris said of her older sister.

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.

Dr. Frank Sulloway, an expert on family dynamics, answered readers' questions about birth order and intelligence. Read his answers. 

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.

It is important to keep in mind, too, that the new study found average difference in I.Q.; the scores varied widely from family to family. In many families, younger brothers and sisters eventually took the lead in I.Q., no matter if they were the screw-up or the whiner.

Moreover, experts have long noted that while even slight differences in I.Q. score can be important for some, the test measures a narrow set of skills. Excessive attention to it can blind parents to the diverse and equally rich expertise that later-born children usually develop.

The best way to react to the news, some psychologists said, is to relax.

“When parents ask me what to do about this, I always say the same thing: nothing,” said Frank J. Sulloway, a psychologist at the Institute of Personality and Social Research at the University of California, Berkeley, and the author of an editorial in the journal Science that accompanied one of the reports. Another report on the study was published in the journal Intelligence.

“Younger siblings are more likely to take chances,” Dr. Sulloway added, and to challenge the status quo in creative ways.

Jackie Orsi, 53, of Morrow, Ohio, grew up the youngest of four, five years behind her nearest sibling, and said she discovered in high school that she scored the highest on I.Q. tests. She remembers the sister closest to her bringing home books from elementary school to read to her.