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. Alfred Adler, 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 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. An issue of ‘Politics and the Life Sciences’ from 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.
Claims about birth order effects on personality have received only mixed support in scientific research. Such research is a challenge because of the difficulty of controlling all the variables that are statistically related to birth order. Family size, and a number of social and demographic variables are associated with birth order and serve as potential confounds. For example, large families are generally lower in socioeconomic status than small families. Hence third born children are not only third in birth order, but they are also more likely to come from larger, poorer families than firstborn children. If third-borns have a particular trait, it may be due to birth order, or it may be due to family size, or to any number of other variables. Consequently, there are a large number of published studies on birth order that vary widely in quality and are inconsistent in their conclusions.
Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order. Ernst and Angst reviewed all of the research published between 1946 and 1980. They found no substantial effects of birth order and concluded that birth order research was a ‘waste of time.’ Contrary to Sulloway’s predictions, they found no significant correlation between birth order and self-reported personality. There was, however, some tendency for people to perceive birth order effects when they were aware of the birth order of an individual. Other studies have supported Sulloway’s claims about birth order. Paulhus and his colleagues found consistent support in self-reports by both student and adult samples. First borns scored higher on conservatism, conscientiousness, and achievement orientation. Later borns scored higher on rebelliousness, openness, and agreeableness. The authors argued that the effect emerges most clearly from studies within families. Results are weak at best, when individuals from different families are compared. The reason is that genetic effects are stronger than birth order effects. Recent studies also support the claim that only children are not markedly different from their peers with siblings. Scientists have found that they share many characteristics with firstborn children including being conscientious as well as parent-oriented.
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. When people are with their parents and siblings, firstborns behave differently than laterborns, even during adulthood. However, most people don’t spend their adult lives in their childhood home. Harris provides evidence that the patterns of behavior acquired in the childhood home don’t affect the way people behave outside the home, even during childhood. Harris concludes that birth order effects keep turning up because people keep looking for them, and keep analyzing and reanalyzing their data until they find them.
Since the 1970s, one of the most influential theories to explain why firstborns frequently score higher on intelligence and achievement tests than other children is the confluence model of Robert Zajonc. This model states that because firstborns mainly have adult influences around them in their early years, they will spend their initial years of life interacting in a highly intellectual family environment. This effect may also be observed in siblings who, although later born, have a sibling at least five years senior with no siblings in between. These children are considered to be ‘functional firstborns.’ The theory further suggests that firstborns will be more intelligent than only children, because the latter will not benefit from the ‘tutor effect’ (i.e. teaching younger siblings). Zajonc’s theory has been criticized for confounding birth order with both age and family size.
The fraternal birth order effect is the name given to the observation 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 the strongest known predictor of sexual orientation, with each older brother increasing a man’s odds of being homosexual by approximately 33%. Even so, the fraternal birth order effect only accounts for a maximum of one seventh of the prevalence of homosexuality in men. There seems to be no effect on sexual orientation in women, and no effect of the number of older sisters.
In the book ‘Homosexuality, Birth Order, and Evolution: Toward an Equilibrium Reproductive Economics of Homosexuality,’ Edward M. Miller suggests that the birth order effect on homosexuality may be a by-product of an evolved mechanism that shifts personality away from heterosexuality in laterborn sons. This would have the consequence of reducing the probability of these sons engaging in unproductive competition with each other. 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. the pressure on newly-wed European aristocrats, especially young brides, to produce ‘an heir and a spare’), and the benefits of encouraging heterosexuality weigh less strongly against the risk of psychological damage that a strongly heteronormative environment poses to a child predisposed toward homosexuality.