Firstborn, middle child, or last-born: Birth order has only very small effects on personality

Who we become only marginally correlates with our birth position amongst siblings. Psychologists from the universities of Mainz and Leipzig, Germany, came to this conclusion in a study recently published in Proceedings of the National Academy of Sciences (PNAS).

The question of whether a person's position among siblings has a lasting impact on personality has occupied scientist for more than 100 years. Laypeople as well a scientist share a number of beliefs: Firstborns are supposedly perfectionists, for example, while middle children develop a talent for diplomacy and last-borns are expected to be rebellious.

To shed some light on the so far inconsistent findings on whether these differences actually exist, Professor Stefan Schmukle and Julia Rohrer of Leipzig University and Professor Boris Egloff of Johannes Gutenberg University Mainz (JGU) analyzed the data of more than 20,000 grown-ups from Germany, the USA, and Great Britain. They found that central personality traits such as extraversion, emotional stability, agreeableness, and conscientiousness are not affected by birth-order position. Only regarding self-reported intellect small effects were found: Firstborns were more likely to report a rich vocabulary and less difficulty understanding abstract ideas.

These self-reports are not completely unfounded as the study confirmed the already known small decline in objectively measured intelligence from first- to last-born. "This effect on intelligence replicates very well in large samples, but it is barely meaningful on the individual level, because it is extremely small. And even though mean scores on intelligence decline, in four out of ten cases the later-born is still smarter than his or her older sibling," explained Schnukle. "The real news of our study is that we found no substantial effects of birth order on any of the personality dimensions we examined. This does not only contradict prominent psychological theories, but also goes against the intuition of many people."

The study was made possible by multiple large longitudinal studies: the German Socio-Economic Panel (SOEP) of the German Institute for Economic Research, the National Longitudinal Survey of Youth (NLSY) of the U.S. Bureau of Labor Statistics, and the National Child Development Study (NCDS) at the Centre of Longitudinal Studies of the University of London.