Birth order

[3] Nevertheless, the notion that birth-order significantly influences personality continues to have a strong presence in pop psychology and popular culture.

[4][5] 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.

[7] In his 1996 book Born to Rebel, Frank Sulloway suggested that birth order had powerful effects on the Big Five personality traits.

[8] However, critics such as Fred Townsend, Toni Falbo, and Judith Rich Harris, argue against Sulloway's theories.

Subsequent large independent multi-cohort studies have revealed approximately zero effect of birth order on personality.

[10] In their book Sibling Relationships: Their Nature and Significance across the Lifespan, Michael E. Lamb and Brian Sutton-Smith argue that as individuals continually adjust to competing demands of socialization agents and biological tendencies, any effects of birth order may be eliminated, reinforced, or altered by later experiences.

Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order.

"[13] More recent research analyzed data from a national sample of 9,664 subjects on the Big Five personality traits of extraversion, neuroticism, agreeableness, conscientiousness, and openness to experience.

[16] In her review of the research, 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.

[18] Robert Zajonc argued for a "confluence" model in which the lack of siblings experienced by firstborns exposes them to the more intellectual adult family environment.

The theory further predicts that firstborns will be more intelligent than only children, because the latter will not benefit from the "tutor effect" (i.e. teaching younger siblings).

[2] The fraternal birth order effect is the name given to the theory that the more older brothers a man has, the greater the probability is that he will have a homosexual orientation.

The fraternal birth order effect is said to be the strongest known predictor of sexual orientation, with each older brother increasing a man's odds of being gay by approximately 33%.

Evolution may have favored biological mechanisms prompting human parents to exert affirmative pressure toward heterosexual behavior in earlier-born children: As more children in a family survive infancy and early childhood, the continued existence of the parents' gene line becomes more assured (cf.

[28] Not all studies, including some with large, nationally representative samples, have been able to replicate the fraternal birth order effect.