A few years ago I wrote about the effect of birth order on personality, a topic of particular interest to me as the youngest of three boys.
At the time I noted that birth order has been studied since early last century, when psychologist Alfred Adler suggested that an only child may become over-protected and spoiled; a second child is often very competitive, wanting to overtake the first child; a middle child may feel insignificant and react by either being even-tempered or a fighter against injustice; and a youngest child always wants to be bigger than the others, may come up with huge plans that seldom work out, and may well be spoiled.
More recent studies (I wrote) have found that only children and first-born children tend to have more cognitive and analytical interests, while later-born children tend to be more artistic and outdoor-oriented; that first-borns tend to be more highly motivated to achieve than later children; and that lastborns are slower at accepting responsible roles, probably because they’ve never experienced being older and more capable than someone else in their family.
I can live with all of that. But the latest study into birth order I have a hard time with: it says that firstborn children score significantly higher in IQ tests than their younger siblings.
Petter Kristensen of the University of Oslo in Norway and colleagues reviewed data collected from 250,000 young men, aged 18 to 19, who took IQ tests as part of their conscription into mandatory military service.
The researchers cross-referenced that data with the Norwegian birth registry to find out whether the men had older or younger siblings, and whether their siblings died shortly after birth or at a relatively early age.
Their analysis showed that firstborn men (at least firstborn Norwegian men) have an IQ about 2.3 points higher than second born–and that the trend continues, so that second-born men have higher IQs than third-born, and so on.
Researchers in the U.S. say that a difference of 2.3 IQ points could translated into a difference of 30 points on an SAT score–which may not sound like much, since the top possible SAT score is 1600, but could be enough to determine whether someone gets into a top-ranked university.
The Norwegian researchers also studied about 600 families with at least four children, and found that the eldest sibling typically had an IQ 2.9 points higher than the fourth-born.
Another way of looking at the data, the researchers say, is that the eldest child in a family has a 13-percent higher chance of having above-average intelligence than the second-born.
They checked to see if the men they were studying had elder siblings who died at a young age because they wanted to be able to determine if it was actual birth order or social birth order that mattered. They found that second-born men whose elder sibling died at a young age, making them socially first-borns, scored, on average, 2.3 points higher on IQ tests than other second-borns whose elder siblings survived.
Having collected this data, of course, the researchers needed to account for it. They came up with a couple of hypotheses. One is that older siblings might have a higher IQ because they have to act as surrogate parents (a.k.a. “they get stuck babysitting their kid brothers/sisters”), tutoring their siblings in various ways.
Another possibility is that parents have more time and resources to invest in their firstborn (which explains why my oldest brother’s baby book has a lot more stuff in it than mine does, I suspect–not that I’m bitter). Among other things, the firstborn may spend more one-on-one time with his or her parents, being exposed to a more sophisticated vocabulary.
Scientists suggest that parents might be able to boost their later children’s IQs by striving to spend more individual time with them.
At this point, I would normally try to sum up my science column with a pithy observation of my own, but frankly, it’s all so complicated I’m not sure I understand any of what I just wrote.
I guess I’d better get my big brothers to explain it to me.