There are a number of theories about birth order. The second child is taller, the first child is bolder, etc, etc. There has long been debate over the connection between I.Q. And birth order. After a large study, scientists have concluded that first born children tend to have a higher I.Q.
The difference in I.Q. Score was very slight--only about 3 points higher in the first born child compared to their closest sibling--but still significant. They also determined that this was due to environmental factors in the home rather than biology. Scientists have also long said that eldest children were more cautious and dutiful than their siblings.
While three I.Q. Points does not seem like that much, it can be the difference between a high B average and a low A average. And over time, this can have a much larger cumulative effect. There are number of theories as to why this is the case. One is that first born children have their parents undivided attention when they are babies. This may enrich their vocabulary and reasoning abilities.