The great debate: Birth order and IQ
TUESDAY, APRIL 28, 2015 11:10 AM

One of the longest-running debates regarding IQ scores is how they may or may not be affected by birth order. In other words, are firstborn children genetically more intelligent than their younger siblings? Though some research may point to this conclusion, many scientists say it's not the case. Read on for more on how birth order affects intelligence:

Are older kids smarter?
One of the largest and most notable studies surrounding birth order and its relation to intelligence was conducted in Norway. The results, published in 2007 in the scientific journal Intelligence, found that of the more than 250,000 siblings studied, the oldest children had average IQ scores of three points higher than those of the youngest children. The research was confined to male siblings and only studied Norwegian families, but the results were relatively consistent across the board. 

Biology vs. environment
What is it about birth order that affects a person's IQ score, though? A 2011 paper published in the Oxford journal CESifo analyzed birth order research and found that, while firstborns had higher IQ scores on average than their later-born siblings, just 3 percent of that was based on biological factors. Instead, what may cause the variation is the environmental differences that occur within the household as more siblings are born. If you consider that firstborn children get 100 percent of their parents' attention before other siblings are born, that could have a major affect on their early development - a time when the brain is growing rapidly. Later-born children won't get that same level of attention during early development because they'll be competing with their older sibling, meaning they may not develop intellectual skills quite as quickly. This aligns with other research, including studies that look at how IQ is affected by adoption, that claims that the environment and rearing have a much larger affect on IQ scores than biological factors.

Birth order and personality differences
It's not just a person's IQ score that may be affected by birth order, though. There is also research that indicates birth order may affect a child's personality. In general, firstborns are thought to be more conscientious, cautious and are natural leaders, while their younger siblings are often considered more outgoing and laid-back. 

Despite the ample research accompanying birth order and its relationship with IQ scores and personality traits, there is still debate surrounding just how strong a role biology plays in these factors. Though the large study from Norway in 2007 convinced many that IQ and birth order are related, it's difficult for researchers to nail down exactly why that might be.