Personality stability and change in sibling status in the one- and two-child-family are examined in this Study. Q-sort data were analyzed for the same 33 male and 34 female subjects during four periods of their lives-- early and late adolescence and early and middle adulthood. Results indicate that stability of personality was greatest during adolescence, followed by adulthood and then late adolescence/young adulthood. Greatest life-long stability in personality occurred for only-child males and only-child females as compared to those with siblings. For those subjects with siblings, birth order was more influential than the sex of the sibling in determining personality stability. Specifically, personality stability was greatest for first born girls and second born boys. These sibling influences were most evident in adolescence. (Author/MP)