Wading into an age-old debate, researchers have found that firstborn children are smarter than their siblings – and the reason is not genetics but the way their parents treat them, according to a study published today.

The study of 240,000 Norwegian men in the journal Science found the IQs of firstborns were two to three points higher than their younger siblings.

While that may not sound like much, experts said even a few IQ points can make a big difference over the course of a lifetime – and set firstborns on a trajectory for success.

University of California, Berkeley researcher Frank J. Sulloway, who wrote a commentary accompanying the study, said two to three IQ points could translate to an added 20 to 30 points on an SAT college entrance exam.

The research is the latest twist in a phenomena that scientists have long noticed but have been at a loss to explain.

Year after year, more Nobel Prizes go to firstborn scientists and authors. Firstborns garner more than their share of National Merit scholarships, and American colleges are filled with disproportionate numbers of firstborn kids.

Lead author Dr. Petter Kristensen, an epidemiologist at the University of Oslo surmised that older children are showered with attention early in life and treated as leaders within the family. They are handed more responsibility after younger siblings are born and live with higher expectations from their parents.

Kristensen said it was clear the results don’t apply to all families and that the odds of the first child being smarter are not overwhelming.

Among brothers with unequal IQ scores, there was a 56.7 percent probability that the oldest brother would score higher, he said.