For many families, birth order holds a significant impact on the molding of children personality and intellect. In the cases of children personality, a phenomenon called de-identification has the most impact with siblings.
According to this phenomenon, siblings try to stand out in a family by first observing what and older sibling does, then doing the exact opposite. Stereotypically, a firstborn tends to be more responsible and intellectual than the others. If following de-identification, the second sibling may take a slacker route. Then, the third sibling would de-identify with the second sibling, opting for a more serious life.
A Chinese study done in the 1990s showed the phenomenon of de-identification as a zigzag pattern (Figure 3), with every odd children scoring higher than the even children. For a while, many researchers have suspected that the pressure of the firstborn place leads the firstborn to a more responsible and mentoring life. Many are not wrong. In this past June, a group of Norwegian researchers released a study showing the firstborns are generally smarter than the younger siblings, having on average, a three-point IQ advantage over the second sibling. These three IQ points, may correlate to +15-point difference on the SATs, which makes a big difference with major colleges.