Forer, Birth Order Factor, 128-9
Firstborns (both male and female) followed by other children have more need for approval than laterborns and only children. Therefore, firstborns, in general, have lower self-esteem than later children do. Only children tend to have higher self-esteem than those from any other position in the birth order….
Highest of all in self-esteem is the only male in the family, whether an only child or with one or more sisters. Next in line is the laterborn male who is not pressed to achieve, and thus feels less guilty if he fails. Since firstborns depend more upon external opinions than laterborns do, the conflict between self-esteem and need for approval is intensified if the firstborn is also a woman. Female laterborns seem to have higher self-esteem than oldest girls, but sex difference continues to matter, because a younger sister with an older brother usually has a lower opinion of herself than a girl with an older sister.