翻译原文：

The eldest children in families tend to develop slightly higher I.Q.s than their younger siblings, researchers are reporting, based on a large study that could effectively settle more than a half-century of scientific debate about the relationship between I.Q. and birth order.

The difference in I.Q. between siblings was a result of family dynamics, not biological factors like changes in gestation caused by repeated pregnancies, the study found.

Researchers have long had evidence that first-borns tend to be more dutiful and cautious than their siblings, early in life and later, but previous studies focusing on I.Q. differences were not conclusive. In particular, analyses that were large enough to detect small differences in scores could not control for the vast differences in the way that children in separate families were raised.

The new findings, which is to appear in the journal Science on Friday, are based on detailed records from 241,310 Norwegians, including some 64,000 pairs of brothers, allowing the researchers to carefully compare scores within families, as well as between families. The study found that eldest children scored about three points higher on I.Q. tests than their closest sibling. The difference was an average, meaning that it showed up in most families, but not all of them.