How big is the effect of birth order on personality?
The effect of birth order on adult personality is small and inconsistent. The most robust evidence comes from a 2023 study of over 14,000 Chinese adults, which found that birth order had no statistically significant or practically meaningful impact on the Big Five personality traits (openness, conscientiousness, extraversion, agreeableness, neuroticism) [4]. This suggests that for broad personality dimensions, birth order alone explains very little of the variation between people.
Other studies find small, specific effects. For instance, a 2022 study of 160 Pakistani students found that last-born girls scored higher on extraversion than first-borns [3]. A 2022 study of 378 MBTI users found that only children were more likely to be introverted, while middle children were less likely to be introverted and more likely to be 'feelers' [5]. However, these effects are modest and context-dependent.
Which specific personality traits are linked to birth order?
When effects do appear, they tend to cluster around social and interpersonal traits rather than core personality. A 2024 study of 304 first-year college students found that firstborns received more parental attention and were more dominant than only children [6]. This aligns with the idea that firstborns often take on leadership roles in the family.
A 2023 study developing the Adler Birth Order Scale identified eight traits linked to birth order: deviant, conscientious, pampered, social interest, parental attention, self-centered, neurotic, and competitive [1]. However, this scale was developed in a specific Pakistani sample and may not generalize. In contrast, a 2022 study of psychotherapists found no significant association between birth order and four dimensions of adult personal self (genial/caring, forceful/exacting, reclusive/remote, ardent/expressive) [2].
What matters more than birth order for adult personality?
Family environment and parenting quality consistently outweigh birth order. The 2022 study of psychotherapists found that perceived family emotional functioning—not birth order—was linked to being more caring and less loudly expressive in adulthood [2]. Similarly, a 2024 study of 252 Filipino college students found that family structure (intact vs. non-intact) and academic program were significantly related to social connectedness, while birth order was not [7].
Even studies that find birth order effects often note that they are partial. The 2024 college study concluded that birth order is 'partially a factor' of personality and social behavior, especially for parental attention and dominance [6]. The researchers recommended considering gender, socioeconomic status, and parenting styles in future research [6]. In short, how you were raised matters more than your position in the sibling lineup.
Sources used in this answer
Development of Adler Birth Order Scale
Developed a 39-item Adler Birth Order Scale with eight traits (e.g., conscientious, pampered, competitive) in a Pakistani sample of 800 participants, showing high reliability (Cronbach's alpha = .90).
From Childhood to Adult Life
Among psychotherapists, birth order and family size showed no significant association with four dimensions of adult personal self; perceived family emotional functioning was more important.
Relationship between Birth Order and Personality Trait (Extroversion)
In 160 Pakistani students aged 16–22, extraversion was significantly correlated with birth order, with last-born girls being more extraverted than first-borns.
Birth order and personality: Evidence from a representative sample of Chinese
In a representative Chinese sample of 14,148 adults, birth order had no statistically significant or meaningful impact on any of the Big Five personality traits.
The Relationship Between Psychological Birth-Order Position and Personality Type
Among 378 MBTI users, only children were more likely to be introverted, and middle children were more likely to be 'feelers' and less likely to be introverted.
Implications of Birth Order on Personality and Social Behavior
In 304 first-year college students, birth order partially affected personality and social behavior: firstborns received more parental attention and were more dominant than only children.
Social Connectedness among Emerging Adults in a State University in Western Visayas
In 252 Filipino emerging adults, academic program and family structure (intact vs. non-intact) were significantly related to social connectedness, but birth order was not.