Editor—Matte et al reported an association between birth weight and childhood IQ.1 To control for confounding by maternal and family factors they examined this relation in sibships of the same sex and found an association between birth weight and IQ within male sibships. This association may be mediated by genetic factors.
The impact of genetic factors on this association can be determined through the investigation of birth weight and IQ in twin pairs. Differences within dizygotic twin pairs are a function of both genetic and non-genetic factors, whereas differences within monozygotic twin pairs are almost completely caused by non-genetic factors.2 If genetic factors mediate the association between birth weight and IQ it is expected that for dizygotic twin pairs the association between intrapair differences in birth weight and IQ is positive, while for monozygotic twin pairs no association is expected.
In a Dutch longitudinal twin study the association between birth weight and IQ was measured in 170 twin pairs of the same sex.3 Birth weight was obtained with a questionnaire, administered to the mother after the birth of the twins. Full IQ was obtained at ages 5, 7, and 10 with the revised Amsterdam child intelligence test (RAKIT), a Dutch intelligence battery, and at age 12 with the Wechsler intelligence scale for children.
Comparison between cotwins with lowest and highest birth weights showed that the dizygotic twins with the lowest birth weight had a lower IQ than their cotwin with the highest birth weight at ages 5 to 10 (table). This difference was not seen in the monozygotic twin pairs. Mean IQ was the same for the twins with the lowest and highest birth weights. When twin pairs with a gestational age of <37 weeks were excluded the results were similar. We also determined the association of intrapair differences in birth weight and IQ. At ages 7 and 10 this association was positive in dizygotic twin pairs (r=0.29, P=0.01; r=0.27, P=0.02) but not in monozygotic twin pairs (r=−0.02, P=0.88; r=0.01, P=0.91).
Our results suggest that genetic factors mediate part of the association between birth weight and childhood IQ, at least until age 10. We found an association between intrapair differences in birth weight and IQ in dizygotic twin pairs. As twin pairs share influences such as prenatal factors, socioeconomic status, parental smoking, and parental age, the influence of these confounders is negligible. In addition, in monozygotic twin pairs, in whom intrapair differences reflect only environmental influences, the association between intrapair differences in birth weight and IQ is absent.
Table.
Full IQ score at ages 5, 7, 10, and 12 of cotwins with lowest and highest birth weights in dizygotic and monozygotic twin pairs. Values are means (SD)
| Dizygotic twin pairs
|
Monozygotic twin pairs
|
||||||||
|---|---|---|---|---|---|---|---|---|---|
| No | Cotwin with lowest birth weight | Cotwin with highest birth weight | P* | No | Cotwin with lowest birth weight | Cotwin with highest birth weight | P* | ||
| All twins | |||||||||
| Birth weight (g) | 2451 (436) | 2804 (380) | 0 | 2337 (427) | 2545 (404) | 0 | |||
| Full IQ score: | |||||||||
| Age 5 | 77 | 99.4 (12.9) | 102.6 (12.8) | 0 | 81 | 102.9 (12.8) | 105.3 (13.4) | 0.01 | |
| Age 7 | 72 | 99.0 (13.9) | 103.2 (14.5) | 0 | 73 | 104.0 (15.1) | 103.7 (14.0) | 0.83 | |
| Age 10 | 75 | 104.1 (13.9) | 106.8 (13.9) | 0 | 75 | 108.0 (16.2) | 107.6 (16.5) | 0.70 | |
| Age 12 | 73 | 98.6 (12.8) | 100.3 (14.1) | 0.22 | 75 | 100.2 (13.9) | 101.27 (12.5) | 0.20 | |
| Gestational age >36 weeks | |||||||||
| Birth weight (g) | 2550 (436) | 2935 (333) | 0 | 0 | 2535 (383) | 2745 (353) | 0 | ||
| Full IQ score: | |||||||||
| Age 5 | 59 | 99.5 (12.7) | 103.0 (11.9) | 0 | 45 | 101.6 (11.7) | 104.7 (13.3) | 0.02 | |
| Age 7 | 56 | 98.2 (14.5) | 104.0 (14.7) | 0 | 42 | 101.1 (14.3) | 101.3 (14.1) | 0.90 | |
| Age 10 | 58 | 104.6 (13.7) | 108.3 (12.2) | 0 | 42 | 106.7 (15.1) | 105.4 (16.1) | 0.34 | |
| Age 12 | 56 | 98.7 (13.2) | 101.73 (13.2) | 0 | 41 | 97.4 (12.4) | 98.3 (11.6) | 0.39 | |
IQ differences between cotwins with lowest and highest birth weights were tested with paired t tests.
References
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