Debate on the role of birth position in personality and intelligence has occupied scientists for over 100 years. In the early 1900s, Alfred Adler stressed the need to understand individuals within their social context, which included, he said, birth order.
According to Adler, first-born children tend to be perfectionists, suffering from neuroses caused by their constant struggle for success and superiority. In contrast, middle children are healthier and easier-going, while last-born children tend to be more rebellious.
Psychologists from the universities of Mainz and Leipzig in Germany analyzed the data of more than 20,000 adults from Germany, the US and the UK. They found that central personality traits such as extraversion, emotional stability, agreeableness and conscientiousness are not affected by birth-order position.
Small effects were found regarding intellect, with firstborns more likely to self-report a rich vocabulary and less difficulty understanding abstract ideas. This replicates results of previous studies. Family environment may influence the variations in intelligence; when there are more adults and fewer children in the home, the overall intellectual environment is richer. This however, is diluted by the arrival of younger children. As the adult-children ratio changes, those born later have less undivided attention from their parents.
Source material from Medical News Today