Study: Older Siblings Have Higher IQs
Two brothers.
Credit: Kati Neudert/dreamstime

Being the oldest child in the family has its perks: later bedtimes, no hand-me-downs, and, according to a new study, a higher IQ.

The study, detailed in the June 22 issue of the journal Science, analyzed the IQs of nearly 250,000 Norwegian 18- and 19-year-old draftees and found that older siblings had higher scores than younger siblings.

Another study, by the same authors of the new Science study but published recently in the journal Intelligence, looked at more than 100,000 Norwegian brothers and found that first-borns on average had an IQ 2.3 points higher than their younger brothers (the IQs were all taken when the brothers were 18 or 19, so they compare the older brother’s score at that age to the younger brother’s score when he reached that same age).

“These are probably the two most important studies on birth order and intelligence in the last 75 years,” said psychologist Frank Sulloway of the University of California, Berkeley, who wrote an analysis of the latest study for Science.

Social order

Unlike earlier studies that compared the intelligence of first-born children to those born later (and that also found that first-borns have higher IQs), the Science study looked at the social order of the children in a family, which does not always correspond to actual birth order in cases where there is a death in the family.

By comparing children who lost an older sibling, for instance, and so were treated as the eldest child, to those who were actually the first-born of their family, the authors showed that the former group had similar IQs to the latter group.

“The second-borns who lost an older sibling are becoming like a first-born” in terms of IQ, Sulloway said.

Sulloway says the new research rules out criticisms of earlier studies that argue that the findings were an artifact of other factors in the data, such as family size and parental IQ.

Reversing trend

Psychologists have a few theories to explain the new results. One proposes that older siblings “tutor” their younger brothers and sisters, which reinforces their own learning, though direct evidence for this particular theory is lacking.

Paradoxically, younger siblings start out in life with higher IQs: Because younger children haven’t yet mastered the skills their older siblings have (for example, language or math skills), they actually degrade the learning environment of their elder brother or sister.

“Every time you add a child, you’re diluting the intellectual environment of everyone in the family,” Sulloway said.

But eventually, around the age of 12, this trend reverses and the older siblings overtake their younger siblings.

Sulloway points out that though older siblings may win out in IQ, the time they devote to studying is time spent elsewhere by their younger siblings, who may excel in other areas such as the arts or sports.