I.Q.'s and birth order
NYT had an article on a Norwegian study on I.Q's and birth order. Most first born children were quite excited about their higher I.Q.'s compared to their siblings. Above is a link to NYT readers questions on the study. I have never been strong on the view of I.Q.'s as being determinats of intelligence.
1.June 24th,
2007
1:00 am What about differences between the second and third or fourth child? Does this three-point gap also apply to the subsequent children?
— Posted by V. Nhan
2.June 24th,
2007
2:00 am One of the most interesting features of the new Norwegian study is that birth-order differences in I.Q. become smaller with increasing birth rank. The difference between a firstborn and a second-born in a family of two children is about 2.3 I.Q. points. The difference between a firstborn and a second-born in a family of three children is 2.1 I.Q. points. By contrast, the difference between a second-born and a third-born in the same family is only 1.1 I.Q. points. In a family of four children, the I.Q. difference between siblings is reduced to only 1 I.Q. point per birth rank, and the difference between the third-born and the last sibling drops to only 0.2 I.Q. points. In general, then, the largest birth-order differences are observed between firstborn siblings and second-born siblings in small families. Children with successively higher birth ranks suffer relatively smaller deficits in I.Q. because of their birth orders.
These intriguing results might be explained by niche partitioning within the family, although this is not the only possible explanation. In two-child families, the firstborn and the second-born may be partitioning their roles into that of the mature and studious achiever and that of the less mature, perhaps more athletic, younger sibling who pursues other types of interests in order to be different (a process called “deidentification”). As other siblings are added to the family system, they appear to position themselves somewhere between these two extremes. Hence the overall difference in I.Q. in a family of five children, between the eldest and the youngest offspring (a total of about 2.7 points), is not much greater than the difference we observe between a firstborn and a second-born in a family of two children (2.3 points).
Theories about the dilution of family resources might also explain why birth order makes less of a difference with higher birth ranks. As more children are added to the family, the relative disparity decreases between the intellectually rich environment of a firstborn, with exclusive access to parental attention before the arrival of other siblings, and the intellectual environment of a family that includes many offspring competing for parental attention.
— Posted by Dr. Frank Sulloway
3.June 24th,
2007
2:01 am Why do some psychologists make such a fuss over a few I.Q. points? In this instance, I think we’re talking about a little more adult attention producing a slightly higher level of verbal and mathematical ability, which might help in school. A pinch of creativity dwarfs that in importance.
— Posted by P. Dorell
4.June 24th,
2007
4:00 am As I noted in the Science Perspective that accompanies publication of the new study by Petter Kristensen and Tor Bjerkedal, the 2.3 I.Q. points that differentiate the average Norwegian firstborn from the average Norwegian second-born in a two-child family is equivalent to the firstborn having a 13 percent greater chance of getting into a better college. This difference is also equivalent to the firstborn having 1.3 times the odds of getting into a better college, compared with the second-born.
It is also worth noting that 2.3 extra I.Q. points (the advantage enjoyed by a firstborn over an immediately younger sibling) is approximately equivalent to scoring an extra 15 points on each SAT test, or a combined 45 points on the three current tests, which have a mean combined score of about 1,500 points. The cutoffs for acceptance to the best colleges, based on SAT scores, often hinge on where one stands within a range of just 40 to 50 points on the three tests combined.
Seen in this perspective, these documented differences in I.Q. by birth order are hardly negligible. However, as I said in a recent interview published in Nature, if I had the choice of having 2.3 extra I.Q. points or having the “enlarged curiosity” that Charles Darwin’s uncle, Josiah Wedgwood, recognized in his nephew on the eve of Mr. Darwin’s departure on the Beagle to circumnavigate the globe, I would unhesitatingly choose the latter.
So, yes, I.Q. is hardly everything, and much that makes people successful in life has to do with how people use their intelligence rather than with their intelligence per se. In addition, there is considerable evidence suggesting that siblings born later use their intelligence differently from the way firstborns use theirs. Indeed, later-born siblings would appear to have 2.3 extra points of one difficult-to-measure intellectual skill, associated with unconventional thinking, that firstborns sometimes lack.
— Posted by Dr. Frank Sulloway
5.June 24th,
2007
5:37 am The problem I have with this study is that it was done in a very homogeneous population. Should not this hypothesis first be tested in other populations before drawing any firm conclusions?
— Posted by P Penko
6.June 24th,
2007
6:55 am This reader raises a very interesting point. Yes, the Norwegian population is probably more homogeneous than, say, the American population. However, one must keep in mind that many well-designed studies of birth order and intelligence — studies that have controlled at minimum for differences in family size and socioeconomic status — have been conducted in America, the Netherlands, France and Israel, among other countries. These studies have consistently shown that firstborn siblings have higher I.Q. scores than second-born siblings, that second-born siblings have higher I.Q. scores than third-born siblings, and so forth.
This newest study from Norway uses a marvelous within-family study design, in which brothers were all compared with other brothers from the same families. It obtains essentially the same results that were found in another large Norwegian sample of individuals who grew up in different families. In short, the results strongly suggest that the findings from previous between-family studies were actually giving us an accurate portrait of birth-order differences in intelligence, as long as these previous studies adequately controlled for differences in family size and socioeconomic status. So we do have reasonably convincing data, from many different countries, that confirm the relationship between birth order and intelligence.
— Posted by Dr. Frank Sulloway
7.June 24th,
2007
7:00 am Is age of the parents a factor?
— Posted by R. Doherty
8.June 24th,
2007
8:00 am Studies have shown the mother’s age at the birth of her first offspring to be a significant predictor of the child’s I.Q. More educated mothers and mothers with higher I.Q. scores tend to have smaller families, and these mothers also tend to bear their first children at later ages than other mothers. As shown in the Norwegian study, differences in I.Q. between firstborns and second-borns were largest among the most highly educated mothers. Hence we would expect birth-order differences in I.Q. to be larger among mothers who have their first child at a later age, since such mothers are likely to be more educated than other mothers.
— Posted by Dr. Frank Sulloway
1.June 24th,
2007
1:00 am What about differences between the second and third or fourth child? Does this three-point gap also apply to the subsequent children?
— Posted by V. Nhan
2.June 24th,
2007
2:00 am One of the most interesting features of the new Norwegian study is that birth-order differences in I.Q. become smaller with increasing birth rank. The difference between a firstborn and a second-born in a family of two children is about 2.3 I.Q. points. The difference between a firstborn and a second-born in a family of three children is 2.1 I.Q. points. By contrast, the difference between a second-born and a third-born in the same family is only 1.1 I.Q. points. In a family of four children, the I.Q. difference between siblings is reduced to only 1 I.Q. point per birth rank, and the difference between the third-born and the last sibling drops to only 0.2 I.Q. points. In general, then, the largest birth-order differences are observed between firstborn siblings and second-born siblings in small families. Children with successively higher birth ranks suffer relatively smaller deficits in I.Q. because of their birth orders.
These intriguing results might be explained by niche partitioning within the family, although this is not the only possible explanation. In two-child families, the firstborn and the second-born may be partitioning their roles into that of the mature and studious achiever and that of the less mature, perhaps more athletic, younger sibling who pursues other types of interests in order to be different (a process called “deidentification”). As other siblings are added to the family system, they appear to position themselves somewhere between these two extremes. Hence the overall difference in I.Q. in a family of five children, between the eldest and the youngest offspring (a total of about 2.7 points), is not much greater than the difference we observe between a firstborn and a second-born in a family of two children (2.3 points).
Theories about the dilution of family resources might also explain why birth order makes less of a difference with higher birth ranks. As more children are added to the family, the relative disparity decreases between the intellectually rich environment of a firstborn, with exclusive access to parental attention before the arrival of other siblings, and the intellectual environment of a family that includes many offspring competing for parental attention.
— Posted by Dr. Frank Sulloway
3.June 24th,
2007
2:01 am Why do some psychologists make such a fuss over a few I.Q. points? In this instance, I think we’re talking about a little more adult attention producing a slightly higher level of verbal and mathematical ability, which might help in school. A pinch of creativity dwarfs that in importance.
— Posted by P. Dorell
4.June 24th,
2007
4:00 am As I noted in the Science Perspective that accompanies publication of the new study by Petter Kristensen and Tor Bjerkedal, the 2.3 I.Q. points that differentiate the average Norwegian firstborn from the average Norwegian second-born in a two-child family is equivalent to the firstborn having a 13 percent greater chance of getting into a better college. This difference is also equivalent to the firstborn having 1.3 times the odds of getting into a better college, compared with the second-born.
It is also worth noting that 2.3 extra I.Q. points (the advantage enjoyed by a firstborn over an immediately younger sibling) is approximately equivalent to scoring an extra 15 points on each SAT test, or a combined 45 points on the three current tests, which have a mean combined score of about 1,500 points. The cutoffs for acceptance to the best colleges, based on SAT scores, often hinge on where one stands within a range of just 40 to 50 points on the three tests combined.
Seen in this perspective, these documented differences in I.Q. by birth order are hardly negligible. However, as I said in a recent interview published in Nature, if I had the choice of having 2.3 extra I.Q. points or having the “enlarged curiosity” that Charles Darwin’s uncle, Josiah Wedgwood, recognized in his nephew on the eve of Mr. Darwin’s departure on the Beagle to circumnavigate the globe, I would unhesitatingly choose the latter.
So, yes, I.Q. is hardly everything, and much that makes people successful in life has to do with how people use their intelligence rather than with their intelligence per se. In addition, there is considerable evidence suggesting that siblings born later use their intelligence differently from the way firstborns use theirs. Indeed, later-born siblings would appear to have 2.3 extra points of one difficult-to-measure intellectual skill, associated with unconventional thinking, that firstborns sometimes lack.
— Posted by Dr. Frank Sulloway
5.June 24th,
2007
5:37 am The problem I have with this study is that it was done in a very homogeneous population. Should not this hypothesis first be tested in other populations before drawing any firm conclusions?
— Posted by P Penko
6.June 24th,
2007
6:55 am This reader raises a very interesting point. Yes, the Norwegian population is probably more homogeneous than, say, the American population. However, one must keep in mind that many well-designed studies of birth order and intelligence — studies that have controlled at minimum for differences in family size and socioeconomic status — have been conducted in America, the Netherlands, France and Israel, among other countries. These studies have consistently shown that firstborn siblings have higher I.Q. scores than second-born siblings, that second-born siblings have higher I.Q. scores than third-born siblings, and so forth.
This newest study from Norway uses a marvelous within-family study design, in which brothers were all compared with other brothers from the same families. It obtains essentially the same results that were found in another large Norwegian sample of individuals who grew up in different families. In short, the results strongly suggest that the findings from previous between-family studies were actually giving us an accurate portrait of birth-order differences in intelligence, as long as these previous studies adequately controlled for differences in family size and socioeconomic status. So we do have reasonably convincing data, from many different countries, that confirm the relationship between birth order and intelligence.
— Posted by Dr. Frank Sulloway
7.June 24th,
2007
7:00 am Is age of the parents a factor?
— Posted by R. Doherty
8.June 24th,
2007
8:00 am Studies have shown the mother’s age at the birth of her first offspring to be a significant predictor of the child’s I.Q. More educated mothers and mothers with higher I.Q. scores tend to have smaller families, and these mothers also tend to bear their first children at later ages than other mothers. As shown in the Norwegian study, differences in I.Q. between firstborns and second-borns were largest among the most highly educated mothers. Hence we would expect birth-order differences in I.Q. to be larger among mothers who have their first child at a later age, since such mothers are likely to be more educated than other mothers.
— Posted by Dr. Frank Sulloway
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