The present study analyzes the differential contribution of a familial or social focus in imaginative ideation (the personal fable and imagined audience mental constructs) to the separation–individuation process of firstborn, middleborn, and lastborn children. A total of 160 young adults were divided into 3 groups by birth order. Participants’ separation–individuation process was evaluated by the Psychological Separation Inventory, and results were cross-validated by the Pathology of Separation–Individuation Inventory. The Imaginative Ideation Inventory tested the relative dominance of the familial and social environments in participants’ mental constructs. The findings showed that middleborn children had attained more advanced separation and were lower in family-focused ideation and higher in nonfamilial social ideation. However, the familial and not the social ideation explained the variance in the separation process in all the groups. The findings offer new insights into the effects of birth order on separation and individuation in adolescents and young adults.