A study published in the Journal of Research in Personality examined the association between birth order, personality, and intelligence, concluding there are minimal associations, with an average absolute correlation of .02 for personality and .04 for intelligence.
Early debates between Freud and Adler highlighted different views on the psychological effects of birth order, with Adler suggesting that birth order impacts neuroses and personality traits. This scientific feud with Freud led to his departure from the Psychoanalytic Society and the birth of the Society for Individual Psychology.
Modern theories grounded in evolutionary psychology suggest that siblings compete for parental attention and resources, which influences their personality development. For example, firstborns are thought to be more conscientious and neurotic, with a goal of pleasing their parents, while laterborns might be more rebellious and sociable.
Birth order has been proposed to affect intelligence as well; this idea traces back to Sir Francis Galton’s research which found an over-representation of firstborn sons among prominent English scientists, an observation that was attributed to greater family investment in firstborns. Modern theories, like the confluence model, suggest that the intellectual environment of a household dilutes with the addition of each child, potentially leading to lower intelligence in laterborns.
In this work, Rodica Damian and Brent Roberts examined the relationship between birth order, personality, and intelligence with three methodological advantages. First, they used a large, representative sample of U.S. high school students; second, they considered a range of sociodemographic factors, such as age, sex, sibship size, parental socioeconomic status, and family structure; third, they focused on personality and intelligence within this sample, allowing for a direct comparison of effect sizes.
Data was sourced from Project talent, a longitudinal study beginning in 1960, involving over 440,000 U.S. high school students. Applying exclusion criteria, such as response credibility, twins, and only children, left a final sample size of 272,003 individuals. Measures of birth order (e.g., “How many of your brothers, half-brothers, foster-brothers, or stepbrothers are older than you?”), family background (e.g., ‘‘With whom are you living; that is, who are the heads of the house?’’), as well as demographic factors were included. Personality was measured via the Project Talent Personality Inventory which included 150 items, from which ten different scale composites were derived, including vigor, calmness, mature personality, impulsiveness, self-confidence, culture, sociability, leadership, social sensitivity, and tidiness. Various tests of cognitive abilities, such as verbal (e.g., reading comprehension), mathematical (e.g., arithmetic reasoning), and spatial skills (e.g., three-dimensional spatial visualization), were evaluated for an intelligence metric.
Damian and Roberts observed remarkably small effect sizes between birth order and personality traits, with the average absolute association being .02, suggesting a minimal effect of birth order on personality development, if there is an impact at all. Within the context of such small effects, the researchers found that compared to laterborns, firstborn children tended to be slightly more conscientious, less sociable, and higher in dominance; further, they tended toward higher levels of agreeableness and lower levels of neuroticism. These findings did not vary across demographics or family background factors.
The study found support for the confluence model, which predicts higher intelligence for firstborns, suggesting that firstborn children benefit from more verbal stimulation and parental attention. The researchers found a slightly larger partial correlation between birth order and verbal ability, compared to math or spatial ability. This finding aligns with the model’s emphasis on the significance of early childhood verbal interactions. Importantly, the effect sizes were quite small, with the largest difference amounting to approximately 1 point on an IQ test. This suggests that while firstborns may experience an advantage, it is not a substantial one.
One limitation to this research is the reliance on self-reported data and inability to control for all possible confounds. Future research can consider comprehensive, longitudinal studies using a variety of reporting methods and extensive control variables.
Overall, given the small effect sizes, Damian and Roberts conclude that birth order does not appear to be critical in shaping personality traits or intelligence. Factors such as parental socioeconomic status and gender produce more meaningful impacts.
The research, “The associations of birth order with personality and intelligence in a representative sample of U.S. high school students”, was published October 2015.
