Another suggestion is that children tend to compare themselves to their siblings. Therefore, firstborn children would benefit from being the oldest, probably the biggest, and the first to reach many milestones. While conflicting data does exist, and further research is necessary, the empirical evidence in these studies shows that firstborn children tend to have higher self-esteem than later born children. Birth order can affect an individual's personality development. Parents may treat older children differently from younger children.
Older siblings exhibit personality traits that differ from their younger siblings.
Specifically, firstborn children tend to have higher self-esteem than laterborn children. Relationships between birth category, achievement, and interpersonal orientation. Birth order and its relationship to depression, anxiety, and self-concept test scores in children. Birth order, perceived appraisals by significant others, and self-esteem.