For centuries, humans have been curious about the effect that birth order has on personality, possibly because eldest children in ruling families typically inherited the seat of power. A new study published in PNAS finds that firstborn children tend to score higher on objective measures of intelligence and self-reported measures of intelligence. But it finds that there are no birth-order effects on other personality characteristics.

Though the findings regarding intelligence are consistent with existing data on birth order, the other results contradict both scientific and common ideas about how much birth order influences personality.

The study in question used data from three large national panels in the US, the UK, and Germany, with a total of more than 20,000 participants. Due to the large size of this data set, the researchers were able to identify even very small effects of birth order on personality with high statistical power.

The researchers determined the subject’s birth order by establishing their position among children in a household, meaning that if a child was raised as the eldest or second-born child in the family, they were treated as such, regardless of whether or not they had other siblings who did not live in the home. The researchers chose this tactic because birth-order effects are believed to stem from the social environment in the home, rather than from actual biological markers that are different for first-born or second-born children. This data came primarily from data on household composition and responses provided in parental questionnaires.

Children without siblings, children with twins, and children from families with more than four children were excluded from the analysis, which primarily focused on children from homes with two to four children. Personality was assessed using self-report measures on personality scales. Intelligence was assessed using self-report measures and by intelligence testing using standard intelligence assessment tools. The five-item measure of self-reported intellect included items such as identification with the sentences “I use difficult words,” “I have a rich vocabulary,” and “I am quick to understand things.”

Once all the data was collated, the researchers ran ordinary least-squares regression analyses to determine whether personality features or intelligence were related to birth order, family size, or country of residence. This analysis allowed them to estimate the unique effect of each family position on personality, rather than assess only an overall linear effect. They also assessed differences within the families in the study using regression models.

The data revealed a strong effect of birth order on intelligence, which was expected. The predominant social and psychological theory is that parents have more emotional and intellectual resources to give the first child (at least until a second child arrives); this boosts the intelligence of the firstborn.

In contrast to this, the data revealed no significant effects of birth order on the Big Five personality traits: extraversion, emotional stability, agreeableness, conscientiousness, openness to experience. They did find a slight effect of openness to experience in the data set from the UK, in that children born later in the birth order were less open to experiences. But this effect wasn’t seen in the other two countries, so it’s hard to draw a definitive conclusion—it may simply be an artifact or a peculiarity of UK culture.

The results run counter to the idea that second-born male siblings in families with firstborn sons would have more female personality characteristics. They also fail to support the idea that later-born children have a greater tendency to extraversion and/or agreeableness.

In terms of the study’s positive findings, the data showed a decline in intelligence related to birth order—children born earlier in the birth order showed higher intelligence than those born later. This effect was true of objective measures of intelligence and also of self-reported measures of intelligence. This finding suggests that later-born children not only receive less intellectual stimulation from their familial environments, but they also receive and internalize messages that they are less intellectual than their older siblings.

Due to the extremely large sample included in this study, the investigators were able to achieve a statistical power of 95 percent for their positive and negative findings. Even more strikingly, a post-hoc analysis of the findings related to intelligence revealed greater than 99 percent statistical power, which means that this finding is more than 99 percent likely to be due to the existence of an actual effect, rather than a random statistical fluctuation.

(A side note: typically, post-hoc analysis is only done on positive findings to ascertain their strength, which is why only intelligence was handled this way.)

Overall, this study used clever statistical mechanisms and an extremely large data set to draw very powerful conclusions about the effect of birth order on personality, a phenomenon that is typically only addressed in theory, rather than using empirical data.

The study’s authors concluded that, aside from intellectual effects, birth order has no effect on personality traits. Therefore, those of us who are unhappy with the arm-chair psychologists who have diagnosed us based on our birth order placement can rest assured that those assumptions that younger children will be more extroverted or agreeable are simply wrong. We have the science to back it up.

PNAS, 2015. DOI: 10.1073/pnas.1506451112 (About DOIs).