First-borns likely have higher IQs than younger siblings

Brothers Ben and Casey Affleck
Brothers Ben and Casey Affleck

A new study has added weight to a long-standing belief that birth-order plays a significant part in sibling intelligence.

Researchers found that, on average, first-borns will have the highest IQ among his or her siblings.

The study, published by psychologists from Leipzig and Mainz, analysed central personality traits of over 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 Professor 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."

Some of the reasons for the findings are explained by the researchers. 

“For me, the most convincing hypothesis is that the parents’ attention is greater and intellectual stimulation is higher if there is only one child,” Schmukle says.

“First-borns get stimulated more, especially verbally, because, maybe, the parents have more time for them,” Professor Rodica Damian told Yahoo Parenting.

“The more siblings in the family, the more the intellectual environment gets diluted. As a first-born, you only talk to adults — they have a large vocabulary, they talk about more high-level ideas. When there are five children, most of your time as a child will be spent talking to other children, who have less intellectual ideas to share.

Instead of learning words from your parents, you are learning words from your brothers or sisters.”