If a child of any birth order has a serious medical problem or a physical or mental disability, that child rises either to firstborn status or lastborn status because parental attention is placed on this special child.
The position of a child in the family, whether a firstborn, a middle child, the youngest, an only child, or one within a large family, has some bearing on the child's growth and development.
For example, if there are two sons and the younger is bigger and more athletic, the younger may be treated as a firstborn because parental favor and expectations are higher for this child.
Children learn by watching other children; however, a firstborn or an only child, who has no example to watch, may not excel in other skills, such as toilet training, at an early age.
Likewise, if the younger of two daughters is extremely pretty and her older sister is plain, the younger may either be treated as a favored lastborn or as a high-achieving firstborn.