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updated 8/12/2010 6:54:03 PM ET 2010-08-12T22:54:03

Birth order within families has long sparked sibling rivalry, but it might also impact the child's personality and intelligence, a new study suggests. First-borns are typically smarter, while younger siblings get better grades and are more outgoing, the researchers say.

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The findings weigh in on a long-standing debate: What effect if any does birth orderhave on a person's life? While numerous studies have been conducted, researchers have yet to draw any definitive conclusions.

The results lend support to some previous hypotheses — for instance, that the eldest sibling tends to have higher aptitude. But the study also contradicts other proposed ideas, for example, that first-borns tend to be more extroverted.

The findings shed light on the influence of sibling relationships, which often receives less attention compared with that of the mother-child or father-child relationship, said Tiffany L. Frank, a doctoral candidate at Adelphi University in Long Island, N.Y., who lead the study.

They also suggest some inherent differences between siblings exist, differences that might arise no matter what parents do. "While parents might want to treat each child equally, it's almost impossible," Frank said here at the 118th Annual Convention of the American Psychological Association.

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Sibling rivalries
Most previous studies on the influence of birth order have looked at children from different families. For instance, some studies have looked at U.S. presidents, Nobel Laureates or NASA astronauts to see whether they are mostly first-born children or later born children.U.S. presidents and science Nobel Laureates were found to be overwhelmingly first-borns, as were 21 of the first 23 NASA astronauts. However, these studies cannot take into account influences that arise from children being in the same family, such as the competition that might exist between siblings, Frank said.

In addition, most previous studies have asked subjects to think back to their childhood or adolescence, a method that might lead to inaccuracies if subjects misremember their past.

In the current study, Frank and her colleagues surveyed 90 pairs of siblings in high school. Subjects were asked to report their grades and rank themselves as compared with their siblings on intelligence, work ethic and academic performance. The researchers also obtained academic tests scores and grades to verify the students' own reports.

While the first-born tended to do better on measures of intelligence, the younger siblings had higher overall grade point averages.

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First-borns might score higher on tests because, at some point in their lives, they were only children who were the sole recipients of their parents' attention.

Younger siblings might earn better grades, because they received mentoring from first-borns who already had to tackle certain subjects, the researchers say. Also, later born children might feel extra pressure to be competitive, and might try to out-do their older siblings in the hopes of gaining extra attention from parents.

In a second experiment researchers looked at differences in personality between 76 pairs of siblings in high school. Subjects rated themselves on a series of statements designed to assess personality.

Later born siblings were found to be more extroverted (sociable, outgoing), sentimental, forgiving and open to new experiences than their older siblings. First-borns were found to be more perfectionistic than their younger siblings.

The youngsters might also be more open to new experiences, because they "see the obstacles that their older siblings have overcome and therefore feel more secure in challenging themselves," the researchers say.

Frank conducted the work with Hannah Turenshine and Stephen J. Sullivan of Lawrence High School in Cedarhurst, N.Y.

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