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Although it is conventional wisdom that your birth order affects your personality, it's a hotly-disputed topic among scientists, and in fact my sense is that, if anything, a majority of researchers doubt the existence of birth order effects. Findings have been slippery: one study suggests that, for instance, first-borns are risk-takers, whereas another suggests that they aren't.
Birth Order & Intelligence
One of the most-researched topics has been intelligence: A wide variety of studies have suggested that first-borns have higher IQ scores than later-borns. While not every study has shown this, Bjerkedal and colleagues published in 2007 what seemed to be the definitive proof. They looked at IQ tests for 250,000 Norwegian male conscripts born from 1967 to 1988 -- that's more than 80% of all Norwegian men born in that time period -- and found first-born sons have IQs of about 2.3 points higher than second-born sons.
Because of the size and completeness of this dataset, they were able to rule out various possible confounds in the data that have been sources of controversy in previous studies. For instance, because wealthy, well-educated families rarely have more than two children, simply being a middle child correlates with being less wealthy and having less access to quality education (and health care, etc.). So one might find that middle children have lower IQs, when in fact what you are measuring is not an effect of birth order, but of socio-economic status. Bjerkedal and colleagues were able to control for such factors.
The Flynn Effect
But, as Satoshi Kanazawa of the London School of Economics points out in a recent paper, there was one confound that they didn't consider: the Flynn Effect. Over the last hundred years -- and possibly longer -- the average person has been doing better and better on IQ tests. In fact, this is something that Bjerkedal and colleagues noticed in their own data, with IQ scores rising slightly from 1984 (the first year of their study) to the mid 1990s.
Because of this, IQ test manufacturers have been constantly raising the bar: you have to get more questions right to get an IQ of 100 now than you did fifty years ago. (What has caused the Flynn effect is one of the Big Questions in current research and a topic for a much longer post.) And Bjerkedal and colleagues did the same thing:
Caveats
There is one limitation to Kanazawa's story. While Bjerkedal and colleagues report that the average score did increase from 1985 through the early 1990s, they report that the scores then decreased back down to the original level between 1998 and 2002 (the study ended in 2004). Also, the increase was very small (one 1 IQ point) compared to the birth order effect that they reported (a drop of 1-2 IQ points for each older brother). So whether the Flynn effect is sufficient to explain away the Bjerkedal results is hard to say.*
Nonetheless, Kanazawa has one more card up his sleeve: his own study, Kanazawa looked at un-scaled data from IQ tests given to 17,419 children in the UK, finding no effect of birth order on intelligence.
That said, the statistical analyses are complicated, involving several transformations. While the transformations seems reasonable (mostly PCA), the transformations Bjerkedal used also seemed reasonable until we realized that they weren't. I'd like to see that Kanazawa's null effect holds up on the truly raw data as well.
Conclusions
Birth order effects are interesting scientifically because they get at the following question: How does your home environment affect the person you become, if at all? Many of the leading minds today suspect that your home environment has little to no effect on you, at least not in the long term. Birth order effects are a very useful test case. Relatively little theoretical rides on whether oldest siblings are the smartest or youngest siblings are the smartest, but if you could show that birth order affected intelligence, that would be a proof-of-concept that home environment affects the adult you become.
[BTW Nobody doubts that home environment has a strong impact on future income, level of educational achievement, etc. The question is whether it affects your personality, making you introverted or extroverted, etc.]
If the intelligence data do not hold up, that leaves -- to my knowledge -- no direct measures of personality or cognitive function for which we have solid evidence that they are affected by birth order. There is one indirect measure that, to my knowledge, has never been challenged: people tend to be friends with and marry others of the same birth order (some of the evidence came from studies run at gameswithwords.org -- thank you to all who participated). Since we know that people marry others with similar personalities (on average), a plausible explanation is that people with similar birth order have similar personalities, leading them to marry one another. However, the fact that no one has thought of another explanation doesn't mean that there isn't one. Time will tell.
See also: My review of birth order effects for SciAm Mind from 2010.
*Bjerdekal and colleagues renormalized a 9-point scaled score. I cannot tell from the article whether than 9-point scale itself was based on standardized norms -- though most likely it was -- and whether those norms were re-standardized during the 21 years of the study.
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Kanazawa, S. (2012). Intelligence, Birth Order, and Family Size Personality and Social Psychology Bulletin, 38 (9), 1157-1164 DOI: 10.1177/0146167212445911Birth Order & Intelligence
One of the most-researched topics has been intelligence: A wide variety of studies have suggested that first-borns have higher IQ scores than later-borns. While not every study has shown this, Bjerkedal and colleagues published in 2007 what seemed to be the definitive proof. They looked at IQ tests for 250,000 Norwegian male conscripts born from 1967 to 1988 -- that's more than 80% of all Norwegian men born in that time period -- and found first-born sons have IQs of about 2.3 points higher than second-born sons.
Because of the size and completeness of this dataset, they were able to rule out various possible confounds in the data that have been sources of controversy in previous studies. For instance, because wealthy, well-educated families rarely have more than two children, simply being a middle child correlates with being less wealthy and having less access to quality education (and health care, etc.). So one might find that middle children have lower IQs, when in fact what you are measuring is not an effect of birth order, but of socio-economic status. Bjerkedal and colleagues were able to control for such factors.
The Flynn Effect
But, as Satoshi Kanazawa of the London School of Economics points out in a recent paper, there was one confound that they didn't consider: the Flynn Effect. Over the last hundred years -- and possibly longer -- the average person has been doing better and better on IQ tests. In fact, this is something that Bjerkedal and colleagues noticed in their own data, with IQ scores rising slightly from 1984 (the first year of their study) to the mid 1990s.
Because of this, IQ test manufacturers have been constantly raising the bar: you have to get more questions right to get an IQ of 100 now than you did fifty years ago. (What has caused the Flynn effect is one of the Big Questions in current research and a topic for a much longer post.) And Bjerkedal and colleagues did the same thing:
To minimize these variations, scores were standardized by calculating deviations from an overall mean score of 5.00 for each calendar year and age.The idea is that your score is based not on how many questions you got right, but how many questions you got right compared with everyone else who took the IQ test in the same year. Kanazawa points out that this is a confound: The average performance was higher in the 1990s than in the 1980s. So if two people who took the test in 1985 and 1995 answered the exact same questions correct, the one who took it in 1995 would have a lower IQ than the one who took it in 1985. This means that if you compare two siblings, the older sibling will -- all else equal -- have a higher IQ score than the younger sibling.
Caveats
There is one limitation to Kanazawa's story. While Bjerkedal and colleagues report that the average score did increase from 1985 through the early 1990s, they report that the scores then decreased back down to the original level between 1998 and 2002 (the study ended in 2004). Also, the increase was very small (one 1 IQ point) compared to the birth order effect that they reported (a drop of 1-2 IQ points for each older brother). So whether the Flynn effect is sufficient to explain away the Bjerkedal results is hard to say.*
Nonetheless, Kanazawa has one more card up his sleeve: his own study, Kanazawa looked at un-scaled data from IQ tests given to 17,419 children in the UK, finding no effect of birth order on intelligence.
That said, the statistical analyses are complicated, involving several transformations. While the transformations seems reasonable (mostly PCA), the transformations Bjerkedal used also seemed reasonable until we realized that they weren't. I'd like to see that Kanazawa's null effect holds up on the truly raw data as well.
Conclusions
Birth order effects are interesting scientifically because they get at the following question: How does your home environment affect the person you become, if at all? Many of the leading minds today suspect that your home environment has little to no effect on you, at least not in the long term. Birth order effects are a very useful test case. Relatively little theoretical rides on whether oldest siblings are the smartest or youngest siblings are the smartest, but if you could show that birth order affected intelligence, that would be a proof-of-concept that home environment affects the adult you become.
[BTW Nobody doubts that home environment has a strong impact on future income, level of educational achievement, etc. The question is whether it affects your personality, making you introverted or extroverted, etc.]
If the intelligence data do not hold up, that leaves -- to my knowledge -- no direct measures of personality or cognitive function for which we have solid evidence that they are affected by birth order. There is one indirect measure that, to my knowledge, has never been challenged: people tend to be friends with and marry others of the same birth order (some of the evidence came from studies run at gameswithwords.org -- thank you to all who participated). Since we know that people marry others with similar personalities (on average), a plausible explanation is that people with similar birth order have similar personalities, leading them to marry one another. However, the fact that no one has thought of another explanation doesn't mean that there isn't one. Time will tell.
See also: My review of birth order effects for SciAm Mind from 2010.
*Bjerdekal and colleagues renormalized a 9-point scaled score. I cannot tell from the article whether than 9-point scale itself was based on standardized norms -- though most likely it was -- and whether those norms were re-standardized during the 21 years of the study.
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