Intelligence, birth order, and family size – Kanazawa – 2012 - Article

What is this article about?

Some research has shown that there is a birth-order effect on intelligence, which means that a child’s birth order among siblings causally influences intelligence. Research on this effect has shown contradictory findings. The confluence model and the resource dilution model both suggest that the average intelligence of children declines with increasing birth order, such that firstborns are on average more intelligent than laterborns. The confluence model states that because firstborns are born in a family with only cognitive mature adults. Secondborns are born into a family that consists of 67% cognitively mature adults, and thirdborns are born in a family that consists of 50% cognitive mature adults. Thus the confluence model states that the birth-order effect is based on the degree of cognitive stimulation in the children’s family environment. The resource dilution model suggests that parents’ material resources, energy, and attention are finite and that with each extra child, there is less of each resource to divide between the children. Therefore, the laterborns (higher-order offspring) are expected to suffer from the lack of resources and therefore attain lower intelligence. The admixture hypothesis suggests that the birth-order effect on intelligence is a methodological artifact of using between-family (cross-sectional) data to make conclusions about within-family dynamics. According to the admixture hypothesis, the negative correlation between birth-order effect on intelligence can be explained by that less intelligent parents are more likely to have a large number of children, and that higher birth-order children come from larger families. Children from smaller families have greater representation among lower birth-order children. The debate over the causal effect of birth order on intelligence is yet to be solved. According to the confluence/resource dilution model, earlierborns are on average more intelligent than laterborns within the same family. The admixture hypothesis contends that children of any birth order from smaller families are on average more intelligent than children of any birth order from larger families.

One empirical finding that emerged is that in studies using between-family data, often a statistically significant effect is found between birth order and intelligence, whereas in studies using within-family data this effect is not found. One Norwegian study found that birth order and sibship have independent and simultaneous effects on intelligence. Boys of a given birth order become less intelligent as their sibship size increases, and sons from the same families become less intelligent as their birth order increases. In a revision of the confluence model, the tutoring effect was included as an explanation. According to the tutoring effect, firstborns initially suffer in intellectual development at the birth of the secondborn, as the secondborn is verbally and intellectually less mature than the firstborn. Later, by being able to tutor the second- and laterborns, the firstborns will recover from the temporary deficit around the age of 12 and will become more intelligent than laterborns after 12. The confluence/resource dilution models and the admixture hypothesis make other predictions. They predict a negative correlation between birth order and intelligence in between-family data. However, the confluence/resource dilution model predicts that birth order has a genuine causal effect on intelligence, and birth order is thus negatively associated with intelligence even net of family size. The admixture hypothesis predicts that the significant association between birth order and intelligence is an artifact and that this will disappear once there is controlled for family size.

What were the methods?

The National Child Development Study (NCDS) is a large, prospective longitudinal study in which a population of British respondents were followed for more than half a century. It included all babies born in Great Britain during 1 week (n = 17,419). They were interviewed in 1965, 1981, in 1999-2000 and in 2004-2005. There were more respondents in Sweep 2 than in the original sample because Sweep 2 included eligible children who were in the country in 1969 but not in 1985 when Sweep 0 interviews were conducted. During each sweep, personal interviews and questionnaires were administered to the respondents, the mothers, teachers, and doctors. The NCDS took multiple intelligence tests at Ages 7, 11, and 16. At the age 7, respondents took four cognitive tests. At age 11, they took five cognitive tests. At age 16 they took two cognitive tests. It was also controlled for social class at birth, mother’s education and father’s education. Another study showed that mother’s lower-body fat increases children’s cognitive ability and shorter birth intervals may deplete the mother’s reserve of such fat. It was therefore also controlled for birth interval.

What can be concluded from this study?

In this article competing predictions from the confluence/resource dilution model and the admixture hypothesis were tested. The analyses indicated that the birth-order effect on intelligence is an artifact of the correlation between intelligence and family size, which is in line with the admixture hypothesis. Once it is controlled for the number of children in a family, the correlation between birth order and intelligence is non-significant. It thus seems that more intelligent individuals are more likely to acquire novel preferences and values, such as having fewer children. It could also be that more intelligent individuals use contraception more effectively than less intelligent individuals. How can one then explain the finding that secondborn sons on average are 2.3 IQ points less intelligent than their firstborn brothers? For findings like these, it is important to consider that the cohorts differed based on historical time period. Over time, individuals become more intelligent and the norms change, so the scores of the laterborns might be lower than that of firstborns. It was thus found that the number of siblings in a family is more important for intelligence than the birth order. This study thus provided evidence for the admixture hypothesis and suggests that the birth-order effect on intelligence is a methodological artifact.