There are numerous factors that influence your child's personality development. While certainly your little one's choices and friendships will affect his behavior and temperament, your family's make-up, the genes that you pass along to him and his familial environment are all powerful influences on your child's personality.
According to the American Academy of Pediatrics, the birth order of your child will affect the way you relate to him, the experiences you share, and -- ultimately -- the way his personality develops. A child with a younger sibling will experience his family differently than the younger child. Dr. Kevin Leman, author of "The Birth Order Book" describes the personalities of first-born children as confident and determined -- eager to please and born to lead -- likely due to the attention they receive before the second child comes along. Last-born children work hard to become the center of attention since they are immediately competing with older siblings. That's why a last-born child is often the family jester and entertainer.
Leman also weighs in on the effects of family size on a child's personality, saying that "only" children -- who spend a lot of time with adults -- are typically confident, well-spoken and have a tendency towards perfectionism. On the flip side, D.H. Sailor, author of "Supporting Children in Their Home, School, and Community" points out that children from large families have more relationships to experience -- which is often both enriching and frustrating. According to a March 1985 research article in "The Journal of Genetic Psychology," children of larger families are more prone to delinquent behavior.
According to a July 2009 article by Jean Mercer, Ph.D. for Psychology Today, biological factors that are present from birth affect a child's personality. Authors of "Child Development, Principles and Perspectives," J.L. Cook and G. Cook Mercer note that researchers studying twins and adopted children to determine the role of nature in development concluded that intelligence, emotions and basic personality traits are 40 to 60 percent a result of genetic inheritance. Genes for certain disorders, such as depression or schizophrenia, might pass down from parent to child, though inheriting these genes from a parent doesn't guarantee that a child will ever develop such a disease.
The environment that you as a family provide for your child will also influence his personality development. According to the AAP, if you and your partner fight in front of your child, he may exhibit aggressive behavior or become withdrawn and cry frequently. A child who is part of a stable family environment develops a sense of belonging -- which leads to a healthy self-esteem, confidence and individuality.