PubMed1992
First borns tend to be different, some of the time, than children born into the other birth orders. This seems due to parental treatment. Parents are often overly anxious about their first child, and may be more restrictive with the first child than with later children. It may be that this is especially true if the first child is female, given the tendency of parents to be more protective of females than of males. Also, the first child has more time alone with the parents than subsequent children, by virtue of having no siblings until the second child is born. From the above factors, it appears that first borns grow up more fearful than later born children but also more intellectually oriented. These are only tendencies and would not be absolutely true in every instance. But, the birth order does seem to initiate a tendency for things to be as stated. As a result, first borns achieve much more than the other birth orders, perhaps due to early adult-oriented styles learned when they had only the parents and no other siblings. If this speculation is correct, the same finding should hold for only children as well. The greater anxiety, achievement, and creativity of some first borns means their lives may be very different as a result of having been the first born child in the family.