The size of a family has a significant effect on the interrelationships among its members and can play a major role in the formation of a child's personality.
Family size is a significant factor in child development, but must be considered as only one part of a larger picture, however. Other factors, such as the parents' personality traits, and the gender and spacing of the children, contribute significantly to the formation of a child's personality. Children of large families have a greater opportunity to learn cooperation at an early age than children of smaller families as they must learn to get along with siblings. They also take on more responsibility, both for themselves and often for younger brothers and sisters. In addition, children in large families must cope with the emotional crises of sibling rivalry, from which they may learn important lessons that will aid them later in life. This factor, however, may also be a disadvantage; either the older child who was "dethroned" from a privileged position or the younger child who is in the eldest child's shadow may suffer feelings of inferiority. Children in large families tend to adopt specific roles in order to attain a measure of uniqueness and thus gain parental attention.
Children in small families receive a greater amount of individual attention and tend to be comfortable around adults at an early age. They may also be overprotected, however, which can result in dependence, lack of initiative, and fear of risk, and the increased parental attention may also take the form of excessive scrutiny and pressure to live up to other people's expectations. Researchers have found that only children are often loners and have the lowest need for affiliation. They tend to have high IQs and are successful academically. However, only children have also been found to have more psychological problems than children from larger families.