A child’s personality is determined both by heredity and by experience. Temperamental differences, for example, are already present at birth. Some babies adapt to the environment outside the womb with ease, while others have a much more difficult time adjusting to the world. These adaptive differences have been found to persist well into adulthood. Hyperactivity and shyness are other temperamental traits that appear early and are observable throughout the life cycle. To be sure such temperamental traits can be modified with effort and training but can never be entirely erased.

Other personality differences, however, are directly related to the child’s experiences in the family. Chief among these is birth order. Research and demographic data (occupational choice and so on) give strong support to the birth order effect. Briefly, the oldest child appears to be the most achievement oriented, and competitive with high self esteem. The middle child tends to be rebellious and insecure. In general the youngest child tends to be the most secure but without the drive to succeed shown by the eldest. These differences hold first and foremost if the children are all of the same sex and if they age difference between them is not more than a few years.

The reason for these differences is fairly straightforward. The oldest child is the first child and heir to all of the parental high expectations and inexperience. In addition, the oldest child has only an adult world of language and behavior to model himself or herself after. When the next child appears, the oldest child is expected to assume some of the care for the younger sibling. Not surprisingly, most astronauts are first born as are many captains of industry and star athletes. While illness or other life circumstances can blunt these birth order derived traits, their pervasiveness speaks to the power of birth order.

Middle children tend to be rebellious and insecure, particularly if they are the only middle children. For one thing, the middle child never has the parents’ exclusive attention. While he or she enjoys a brief period of being the youngest, the child is robbed of that position as soon as the younger sibling is born. That lack of a long period of being at the center of parental attention, adds to the insecurity and often to a life long effort to win back parental affection. Because the middle child is often the odd man between an alliance of oldest and youngest, he or she often feels the outsider. Rebellion against authority is a common consequence.
The youngest child tends to have the best of both worlds. As the last child he or she continues to receive the most parental attention. Moreover with the last child, parents are more experienced, relaxed and permissive. This puts less pressure on the child to achieve, gives him or her a greater sense of security and self-esteem, and a greater licence for the child to follow his or her passions.

Birth order considerations have some practical consequences. As American families, and in other advanced countries, grow smaller the proportion of first-borns and only children (who closely resemble first-borns) will increase. This will alter the composition of children in the classroom, and increase the competitiveness. This is one cause of the current over-programming and over-scheduling of children. Down the road, the best chance for a successful marriage occurs between a first born and last born. Least successful are marriages between two first born or last born individuals. In the latter cases either both partners want to be charge or both want to be taken charge of. Obviously these are not hard and fast realities, and subject to many variations, but they are useful to keep in mind, just in case.