The association of high birth weight with intelligence in young adulthood: a cohort study of male siblings (original) (raw)
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BMJ, 2001
Objective To examine the relation between birth weight and measured intelligence at age 7 years in children within the normal range of birth weight and in siblings. Design Cohort study of siblings of the same sex. Setting 12 cities in the United States. Subjects 3484 children of 1683 mothers in a birth cohort study during the years 1959 through 1966. The sample was restricted to children born at >37 weeks gestation and with birth weights of 1500-3999 g. Main outcome measure Full scale IQ at age 7 years. Results Mean IQ increased monotonically with birth weight in both sexes across the range of birth weight in a linear regression analysis of one randomly selected sibling per family (n = 1683) with adjustment for maternal age, race, education, socioeconomic status, and birth order. Within same sex sibling pairs, differences in birth weight were directly associated with differences in IQ in boys (812 pairs, predicted IQ difference per 100 g change in birth weight = 0.50, 95% confidence interval 0.28 to 0.71) but not girls (871 pairs, 0.10, − 0.09 to 0.30). The effect in boys remained after differences in birth order, maternal smoking, and head circumference were adjusted for and in an analysis restricted to children with birth weight > 2500 g. Conclusion The increase in childhood IQ with birth weight continues well into the normal birth weight range. For boys this relation holds within same sex sibships and therefore cannot be explained by confounding from family social environment.
2006
Background: The mechanisms underlying the observed association of childhood intelligence with body mass index (BMI) are unclear and few studies of this association have been prospective in design. Methods: Prospective study in a birth cohort of 5467 individuals who were born in Aberdeen, Scotland between 1950 and 1956 and who responded to a follow-up survey in 2001. Comparison of associations within sibling pairs of the same family to associations between different families in 643 sibling pairs (1286 individuals) who are participants in the main cohort. Results: Childhood intelligence (age 7 years) and educational attainment were both inversely associated with adult BMI (mean age 48 years): the sex-and age-adjusted mean change in adult BMI per s.d. of intelligence was À0.35 kg/m 2 (95% CI: À0.49, À0.21 kg/m 2) and per unit increase in educational category (seven categories) was À0.28 kg/m 2 (95% CI: À0.34, À0.22). On adjustment for education the association between childhood intelligence and adult BMI attenuated to the null (À0.03 kg/m 2 (À0.19, 0.13 kg/m 2)); other potential confounding or mediating factors had little or only modest effects on this association. The association between education and adult BMI was not affected by adjustment for childhood intelligence or other potential covariates. The within sibling-pair effect of education on adult BMI (À0.06 kg/m 2 (95% CI: À0.26, 0.14)) was weaker than the effect between different families (À0.37 kg/m 2 (95%CI: À0.58, À0.17)), P-value for difference of within sibling and between family effect ¼ 0.03. Conclusions: The association of childhood intelligence with adult BMI is attenuated to the null on adjustment for educational attainment, whereas the association of educational attainment with adult BMI appears to be independent of childhood intelligence and other measured covariates. However, our family analyses suggest that fixed family and neighbourhood factors, which are closely matched in siblings of a similar age, explain much of the association between greater educational attainment and lower adult BMI.
Intelligence and birth order in boys and girls
Intelligence, 2008
The relation between intelligence and birth order was shown in a recent publication . Intelligence test scores and birth order among young Norwegian men (conscripts) analyzed within and between families. Intelligence, 35, 503-514] to be negative. Subjects in this and in an influential earlier study . Birth order, family size, and intelligence. Science, 182, 1096Science, 182, -1101 were all men. We tested if the association of IQ and birth order is the same in men and women. Longitudinal IQ data were available from 626 Dutch twin pairs at ages 5, 12 and 18 years. The number of older siblings in these twin families was between zero and five, and was recoded into 3 categories (0, 1 and 2, or more). IQ data were analyzed with a model in which age cohort, number of older sibs, sex and all interactions were included as fixed effects. The dependency between twins was modeled as a function of additive genetic effects (A) and common environment (C) shared by children from the same family. Effects of A, C and unique environment (E) were allowed to differ as a function of age. The correlation across time between IQ scores was modeled a function of genetic and environmental factors.
Behavior Genetics, 2011
A robust positive correlation between height and intelligence, as measured by IQ tests, has been established in the literature. This paper makes several contributions toward establishing the causes of this association. First, we extend the standard bivariate ACE model to account for assortative mating. The more general theoretical framework provides several key insights, including formulas to decompose a crosstrait genetic correlation into components attributable to assortative mating and pleiotropy and to decompose a crosstrait within-family correlation. Second, we use a large dataset of male twins drawn from Swedish conscription records and examine how well genetic and environmental factors explain the association between (i) height and intelligence and (ii) height and military aptitude, a professional psychogologist's assessment of a conscript's ability to deal with wartime stress.
Birth Weight and Cognitive Ability in Childhood Among Siblings and Nonsiblings
PEDIATRICS, 2008
OBJECTIVES. The purpose of this work was to examine whether the positive association between birth weight and childhood cognitive ability is seen within siblings from the same family, as well as between nonsiblings, and to determine whether these associations vary with age. METHODS. We compared the association of birth weight with cognitive ability measured at ages 5 to 6, 7 to 9, and 11 to 12 years among a total of 5402 children from different families with that among 2236 to 3083 sibships from the National Longitudinal Study of Youth 1979-Children. RESULTS. In the whole cohort, there were positive associations between birth weight and cognitive ability at all ages, with the association increasing with age from a 0.81-point increase at ages 5 to 6 years to 1.30 and 1.44 points at ages 7 to 9 and 11 to 12 years, respectively, per 1 SD of gestational age- and gender-adjusted birth weight z score. With adjustment for covariates, there was marked attenuation of these associations. Mean...
American Journal of Epidemiology, 2013
Parental ages are increasing in the developed world, and postponed parenthood may have a negative association with the cognitive ability of offspring. There is, however, inconclusive evidence regarding the impact of both maternal and paternal ages. We have been able to reduce or eliminate unobserved confounding by using methods that account for fixed parental characteristics shared by brothers. Associations between parental age and intelligence quotient (IQ) among 565,433 Swedish males (birth cohorts 1951 to 1976) were analyzed, with IQ measured at conscription examinations (given between ages 17 and 20 years). When we accounted for the IQ time trend by adjusting for birth year, advanced paternal age showed no association with offspring IQ; however, maternal ages above 30 years were inversely associated with offspring IQ. For example, maternal ages 40-44 years were associated with an offspring IQ that was 0.07 standard deviations lower than that for maternal ages 25-29 years (P < 0.001). However, the IQ trend more than offset the impact of age, as without birth year adjustment, advanced maternal age was positively associated with IQ. Although the results confirmed that maternal age was negatively associated with offspring IQ, the association was small enough that delaying parenthood resulted in higher offspring IQ scores because of the positive IQ test score trend. cognitive ability; intelligence; maternal age; paternal age Abbreviations: IQ, intelligence quotient.
ABSTRACT Older and Wiser? Birth Order and IQ of Young Men
While recent research finds strong evidence that birth order affects children's outcomes such as education and earnings, the evidence on the effects of birth order on IQ is decidedly mixed. This paper uses a large dataset on the population of Norway that allows us to precisely measure birth order effects on IQ using both cross-sectional and within-family methods. Importantly, irrespective of method, we find a strong and significant effect of birth order on IQ, and our results suggest that earlier born children have higher IQs. Our preferred estimates suggest differences between first-borns and second-borns of about one fifth of a standard deviation or approximately 3 IQ points. Despite these large average effects, birth order only explains about 3% of the within-family variance of IQ. When we control for birth endowments, the estimated birth order effects increase. Thus, our analysis suggests that birth order effects are not biologically determined. Also, there is no evidence t...