Is big sister always telling you what's best? Does big brother seem to know it all?

Instead of stewing in resentment, maybe you should start listening when they dispense smart-alecky advice.

Turns out, they really are brighter than you, by and large.

A large study by Norwegian scientists appearing in today's edition of Science, the weekly academic journal published by the American Association for the Advancement of Science, concludes that a child raised as the eldest has a higher intelligence quotient, on average, than younger siblings.

The difference is just a couple of IQ points -- not exactly the gap between Albert Einstein and Homer Simpson.

But the extra smarts conferred by birth order are significant enough in the broad sense. Spread over tens of thousands of individuals, the tiny difference could translate into a higher likelihood of acceptance into better colleges, improved chances of landing a good job, or even better luck at winning a brainy spouse, according to some child experts.

Researchers Petter Kristensen and Tor Bjerkedal make no such bold claims in their study of 241,310 Norwegian 18- and 19-year-olds. The findings of the medical scientists -- Kristensen is an epidemiologist at Norway's Institute of National Health; Bjerkedal is a physician-researcher with the Norwegian Armed Forces Medical Service -- are sure to roil the field of child psychology and family studies.

Slightly better brains appear to be a result of the way the senior child is raised and adapts to the family, not to any special genetic traits that go with coming first, the researchers say.

The study is long on numbers and notably short on interpretation. But the bottom line is blunt: The child who holds position as the oldest will possess, on average, an IQ 2.3 points higher than younger siblings. With a sample size of nearly a quarter-million test subjects, that's compelling scientific evidence.

"This is one of the most important findings in this field to come along in the past 70 years or so," said Frank J. Sulloway, a visiting professor of psychology at the University of California, Berkeley, and advocate of the view that birth order influences personality.

Sulloway spoke in an interview, but also wrote an accompanying commentary in Science on the Norwegian study.

"This is a beautiful study that should put an end to an unnecessarily heated debate over intelligence and birth order," he said. "If you were raised as the first born, you will most likely have a higher IQ -- end of story. Now we should focus effort on finding what this means."

Other specialists were not so sure, however.

The Norwegian study yields "interesting and valuable data, but nevertheless leaves important methodological questions unanswered that raise the same kind of doubts" that scientists have debated in the past, said Joseph L. Rodgers, professor of psychology at the University of Oklahoma.

Rodgers, a well-known doubter of the view that birth order affects intelligence, said in an e-mailed response to Globe questions that the Norwegian study might be fundamentally flawed because of structural problems in the research.

"More description of methodology is required," he said.

At first blush, the Norwegian findings suggest a victory of "nature" over "nurture." Birth order, after all, is basically an accident: You are born first or you aren't.

In fact, however, the research draws an opposite conclusion. It makes no difference if one is biologically the oldest -- what matters is whether an individual is raised as the oldest.

In families where the actual first child dies in infancy or at birth, for example, the next-born child will, on average, possess a higher IQ than younger sisters and brothers. In families where the first two children die very early, the third child is the one who is likely to have extra IQ points.

What that means, the study suggests, is that birth order as a biological fact means far less than the position within a family taken by the child who happens to be oldest.

"The new study absolutely rules out that biology is the main thing," said Sulloway. "Every sibling competes for family favor.

"The oldest child understands that he or she will find the best niche by imitating the parents, by valuing the things they value -- that might be studying harder, or taking on responsibilities, or becoming a surrogate parent by helping out with the snotty youngers."

Younger children, by contrast, may find other ways of securing parental love and attention. They tend not to follow the formula of their older brother or sister -- instead carving new family niches that do not always help boost intelligence, but which win approval, such as being very obedient or excelling at sports.

Intelligence is partly a matter of genes. But it's also heavily influenced by environment and social factors in early development, according to most studies .

The child who strives hardest to be like a parent may glean a few extra IQ points in the process, according to some child psychologists and behavior specialists.

The notion that the oldest child is likely to be the brightest has been around for decades, boosted by such curious coincidences as the fact that firstborns seem unusually prevalent among Nobel laureates, great classical musicians, and US presidents.

The Norwegian study doesn't seek to explain why the oldest child is likely to be smarter.

But it represents by far the largest analysis ever attempted on the effects of birth order on intelligence and seems to provide powerful evidence that the child raised as the oldest somehow acquires an intellectual advantage.

"The mean IQ difference documented between a Norwegian firstborn and a Norwegian second-born is only 2.3 points," Sulloway writes in his commentary. "Such a modest difference can have far greater consequences than most people realize."

For example, he argued, statistical analyses show that if two siblings applied to a prestigious university, the odds of the older child winning a spot would be 1.3 times better, according to his formula.

"Such differences in opportunities gained or lost inevitably accumulate over one's lifetime," Sulloway said in the commentary.