What are the factors that influence your child's personality and other attributes? Two major influences are at work: nature (genetic factors, heredity) and nurture (experience). These interact with each other in ways that are particular to each child. To better understand the similarities and differences among siblings, let's look at both of these factors more closely.
Because of heredity—the biological or genetic influences of the same two parents on each child—parents might expect their children to be alike. But overall, children have only about a fifty-fifty chance of developing any particular inherited trait (physical appearance, personality, intelligence, aptitudes, health), and even when these traits are present, they can vary.
For instance, researchers have found that siblings tend to be more similar in their physical characteristics than in their likelihood of developing the same diseases. Also, while siblings may resemble one another in their intellectual aptitude and other psychological characteristics early in life, these similarities generally diminish by adulthood, while differences become more pronounced. Even in childhood, siblings with similar levels of intelligence may differ in their school achievement, since academic success can be strongly affected by the different life experiences of each child.
Nurture (or experience) refers to the nonhereditary influences on your child's development. They include social factors such as relationships with siblings, peers, parents, and other adults, as well as environmental influences like illnesses, accidents, nutrition, and cultural experiences. Other forces come into play as well, including your child's perceptions of herself and others, past experiences, self-expectations, and the expectations others have of her.
Siblings share some experiences but have many others that are not shared. While shared experiences generally contribute toward similarities, even a shared experience may affect each child differently. And since most experiences are unshared, they contribute to differences between children too.
In the early school years, for instance, qualities such as intelligence and academic achievement are largely determined by heredity and shared experience. However, as children grow, they have more unshared experiences, which gradually help differentiate one sibling from another. Siblings even perceive and interpret shared events differently, and these different perceptions can be important in shaping a child's development and self-image.