Birth order

Birth order refers to the order a child is born in their family; first-born and second-born are examples. Birth order is often believed to have a profound and lasting effect on psychological development. This assertion has been repeatedly challenged; the largest multi-study research suggests zero or near-zero effects. Birth-order theory has the characteristics of a zombie theory, as despite disconfirmation, it continues to have a strong presence in pop psychology and popular culture.

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, causing them to develop a Middle child syndrome. Younger and only children may be pampered and spoiled, which was suggested to affect their later personalities.

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 empirical research shows that birth order does not influence the Big Five personality traits.

...
Wikipedia