Substantial proof: birth order and personality not related. Massive research conducted at the University of Illinois revealed that birth order has no significant effects on IQ or personality. Even though consistent differences were seen between first-borns and their siblings, scientists claim that the variation is practically negligible and has no practical impact on an individual’s life. The findings were published in the Journal of Research in Personality.

Problems With Previous Studies

Most of the previous studies conducted on birth order used small samples. Moreover, many used a ‘within-family design’, comparing children with their siblings only. While some claim that this design is better than a ‘between-subjects’ one, postdoctoral researcher Rodica Damian (now a Professor of Psychology at the University of Houston explained that comparison among siblings does not fully measure the personality of each individual child. Often only the eldest child is questioned about his/her IQ and conscientiousness, which is a misrepresentation of the actual scenario.

Another significant problem with within-family designs is the background belief that the eldest child is more responsible and wise as compared to others. Parents often make this claim, without realizing the obviousness of it – he has those traits because he is the oldest!

A Stable And Detailed Study Design

Researchers claim that an accurate within-family design should follow the families over time, recording data relating to personality and IQ from each child at a specific age. However, Brent Roberts, lead scientist and Professor of Psychology at the University of Illinois, along with Rodica Damian chose a between-families approach. They enrolled 377,000 high school students – the largest in the history of any study relating to birth order and personality.

For a reliable interpretation, any potential confounding variable was controlled, such as socio-economic status of the child’s family, the number of siblings and the relative age of the siblings at the time of study. These factors can give way to askew results; for example, wealthier families have fewer kids (more first-borns only) who have access to better resources and learning facilities (better IQ).

Moreover, the research team looked into a specific subset of children in the study – those who had two siblings and were living with two parents. This allowed them to pinpoint any significant differences between first- and second-borns, or second- and third-borns.

Birth Order And IQ: New And Profound Results

Data analysis revealed what many previous large-scale studies had observed – first-borns had a one-IQ point advantage as compared to their siblings. Despite being statistically significant, Damian claims the observation to be meaningless.

Furthermore, consistent differences between the personalities of first-borns and their siblings were also seen. First-borns were more extroverted, conscientious and agreeable, with fewer anxiety issues. However, these differences were “infinitesimally small” according to Roberts, amounting to a correlation of 0.02.

“In certain cases, for example, if a drug saves 10 out of 10,000 lives, small effects can be profound”, explained Roberts. “However, in terms of personality traits and how you rate them, a 0.02 correlation doesn’t get you anything significant. You are not going to observe this difference with the naked eye if you make two people sit down next to each other and compare them. Also, the magnitude of the differences between first-borns and second-borns, and second-borns and third-borns was minuscule”.

Damian concluded that these results are a message for parents to not change or modify their parenting styles or skills according to birth order, since the latter is not meaningfully associated with personality or IQ.