Birth order is defined as a person's rank by age among his or her siblings. Birth order is often believed to have a profound and lasting effect on psychological development. This assertion has been repeatedly challenged by researchers, yet birth order continues to have a strong presence in pop psychology and popular culture.

Theories

Alfred Adler (1870-1937), an Austrian psychiatrist, and a contemporary of Sigmund Freud and Carl Jung, was one of the first theorists to suggest that birth order influences personality. He argued that birth order can leave an indelible impression on an individual's style of life, which is one's habitual way of dealing with the tasks of friendship, love, and work. According to Adler, firstborns are "dethroned" when a second child comes along, and this may have a lasting influence on them. Younger and only children may be pampered and spoiled, which can also affect their later personalities. Additional birth order factors that should be considered are the spacing in years between siblings, the total number of children, and the changing circumstances of the parents over time.

Since Adler's time, the influence of birth order on the development of personality has become a controversial issue in psychology. Among the general public, it is widely believed that personality is strongly influenced by birth order, but many psychologists dispute this. One important modern theory of personality states that the Big Five personality traits of Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism represent most of the important elements of personality that can be measured. Contemporary approaches to birth order frequently suggest that birth order influences these five traits.

In his book Born to Rebel, Frank Sulloway suggests that birth order has strong and consistent effects on the Big Five personality traits. He argues that firstborns are more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to laterborns. However, critics such as Fred Townsend, Toni Falbo, and Judith Rich Harris, argue against Sulloway's theories. An issue of Politics and the Life Sciences, dated September, 2000 but not published until 2004 due to legal threats from Sulloway (who claimed its content to be defamatory, although it was carefully and rigorously researched and sourced), contains criticisms of Sulloway's theories, including studies that show conflicting findings.

In their book Sibling Relationships: Their Nature and Significance across the Lifespan, Michael E. Lamb and Brian Sutton-Smith make the point that sibling relationships often last an entire lifetime. They point out that the lifespan view proposes that development is continuous, with individuals continually adjusting to the competing demands of socialization agents and biological tendencies. Thus, even those concerned only with interactions among young siblings implicitly or explicitly acknowledge that all relationships change over time and that any effects of birth order may be eliminated, reinforced, or altered by later experiences.