Before you roll your eyes, here is how the economist, Kasey Buckles of the University of Notre Dame, came to that conclusion.
She uses data from the 1979 National Longitudinal Survey of Youth (yes, I know, it is 31 years old, but economists do not always have recent data to work with). It surveyed more than 12,000 people between 14 and 22 years old.
Buckles says an older child with a sibling who is at least two years younger tended to do better in test scores. She surmises that this is because these older children received more attention from their parents in their crucial developmental stages. Those who see a younger sibling arrive at 1.5 years old might not get as much attention because of the presence of a new baby in the house.
For more details from the study click here .