A recent study has suggested that the eldest child in the family is much more likely to be smarter than the others.
"Latter-born children perform worse on cognitive assessments from a very early age, well before they enter school," the researchers say.
"Although latter-born children are not born disadvantaged in their health or developmentally, we find that parents are unable to provide them with the same level of cognitive support as they do with their first-born."
The researchers go on to add that their studies suggest that after having one child, parents become more picking with what information they teach to their kids.
This decision appears to "set their latter-born children on a lower path for cognitive development and academic achievement, with lasting impact on adult outcomes."
So essentially parents have less time to spend with the kids individually which means they don't pass on as much knowledge as they do to their first born.
Source: Research Economics