Those of us who have siblings tend to have mostly friendly rivalries with them. Some, if not most, of us probably believe that we are more intelligent than all of our siblings; however, some research shows that the oldest sibling tends to have the highest aptitude. Now, for all of us who are not the oldest, we shouldn't worry too much. We tend to actually get better grades and be more extroverted. These results were found by surveying 90 pairs of high school siblings. The researchers believe that the oldest siblings tend to be the most intelligent because they received all of their parent's attention for a certain amount of time. They also believe that the younger siblings receive better grades because the older sibling could mentor them.
Some older studies about this particular debate found that the oldest sibling tends to be the most extroverted. This study had adults look back upon their childhood, and some of the people studied may have not remembered their childhood correctly. This study could not be replicated, so it violates the critical thinking principle of replicability.
Many of us may continue to ignore these studies and will still believe that we are the smartest sibling. It is still interesting that studies are able to show the general differences between many siblings.