A study found that firstborn children have higher IQs than younger sibling. The difference was only about three points the difference between a high B average and a low A, for instance. Researchers have long had evidence that firstborns tended to be more dutiful and cautious than their siblings, and some previous studies found significant I.Q. differences. The researchers found that this difference was probably not due to biological factors by also looking at people who had become the oldest after the death of an older sibling. They found the same results for this group. One theory social scientists have about this effect is that the older sibling gets more parental attention before other siblings are born that can never be made up for by other children, who must always share their parents' attention. Older children may also gain from teaching younger siblings, benefitting more from the lessons than their students. Younger siblings may be more likely to excel in other areas not measured by intelligence tests, such as Darwin, Copernicus, and Descartes who all had older siblings.