In one of the few large-scale studies to examine the weight of family effects on IQ, researchers in Norway found data to support that “social rank” — or how children are raised and the roles they take on in their family — explains IQ differences more so than biological factors.
The researchers, led by Petter Kristensen, M.D., Ph.D., M.Sc., of Norway’s National Institute of Occupational Health, suggested that the differences they found were likely explained by interaction within the family.
The theory of how family interaction explains these differences suggests that the first born has the advantage of getting all the attention and resources of the parents until the next child comes along (including, for instance, the financial resources for pre-school and private schooling). Later, the parents’ resources have to be shared with more children.
In the study, Norwegian epidemiologists analyzed data on birth order, health status and IQ scores of 241,310 men, who were 18 and 19 years old, who took an intelligence test as part of Norway’s compulsory military board examination.
To test whether the difference could be due to biological factors, the researchers also examined the scores of young men who became the eldest in the household after an older sibling had died. Their scores came out the same, on average, as those of biological firstborns. This finding suggests that it is family and social factors within the family — not just biology or genetics — that may significantly help influence a person’s intelligence.
After correcting for factors that may affect scores, including parents’ education level, maternal age at birth and family size, the researchers found that eldest children scored an average of 103, slightly higher than second children, 100, and thirdborns, 99. A score of 100 is considered “average intelligence” on most IQ tests.
While these differences are statistically significant for the study, they have little clinical significance in the measurement of intelligence between two individuals, contrary to many mainstream media reports on the study.
IQ is the abbreviation for “intelligence quotient,” the score one receives when taking a specific kind of test designed to measure the degree of a person’s intelligence. Intelligence is a hypothetical psychological construct. Intelligence tests include the physical manipulation of objects as well as questions on a wide variety of subjects.
Biological birth order (which includes all children in a family, including those who have died) and social birth order (which includes all living children in a family) may be equally important with respect to children’s IQ scores, noted Kristensen and his colleagues.
The study was a brief report published in the 6/22/07 edition of Science.
Source: Kristensen, P. & Bjerkedal, T. (2007). Explaining the Relation Between Birth Order and Intelligence. Science, Vol. 316. no. 5832.