They found that first-born children performed better on psychometric intelligence tests, which corresponded to a higher level of intellect when compared to their later-born counterparts who took the same tests. Sulloway’s findings also noted that first-borns showed a higher level of conscientiousness. He hypothesized their increased level of conscientiousness was a result of first-born children often being delegated to act as surrogate parents to their younger siblings. This also instills a certain level of maturity and responsibility first-born children will carry with them well into adulthood.
Let me just say that in my family, this was definitely not true. Then again, my sister (one year younger) was treated more like the first born and she fits this description to a tee.
