Birth order is a person's rank by age among his or her siblings. Typically, researchers classify siblings as “eldest”, “middle child”, and “youngest” or simply distinguish between “firstborn” and “later born” children.

Birth order is commonly believed in pop psychology and popular culture to have a profound and lasting effect on psychological development and personality. For example, firstborns are seen as conservative and high achieving, middle children as natural mediators, and youngest children as charming and outgoing. In his book Born to Rebel, Frank Sulloway argues that firstborns to be more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to laterborns. Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order on personality. [2][3] In her review of the scientific literature, Judith Rich Harris suggests that birth order effects may exist within the context of the family of origin, but that they are not enduring aspects of personality. [4]

Some research has found that firstborn children have slightly higher IQs on average than later born children. [5] However, other research finds no such effect. [6]

In practice, systematic birth order research is a challenge because it is difficult to control for all of the variables that are statistically related to birth order. For example, large families are generally lower in socioeconomic status than small families, so third born children are more likely than firstborn children to come from poorer families. Spacing of children, parenting style, and gender are additional variables to consider.
http://en.wikipedia.org/wiki/Sibling#Birth_order
"Brother" and "Sister" redirect here. For other uses, see Brother (disambiguation) and Sister (disambiguation).

Two Sisters by William-Adolphe BouguereauOne's sibling is one's brother or sister, respectively meaning a male or female with whom one shares at least one parent. This is usually taken to mean that the two people are genetically very close, though it is not always necessarily the case, for example one or more siblings may have been adopted by their parents.

In most societies throughout the world, siblings will usually grow up together and spend a good deal of their time during childhood together. They may have conflicts during their childhood years, but usually resolve them later in life. This closeness may be marked with the development of strong emotional associations such as love and enmity. The sibling bond is often complicated and is influenced by factors such as parental treatment, birth order, personality, and people and experiences outside the family. [1]
http://en.wikipedia.org/wiki/Sibling