Twins and the fetal origins hypothesis: An application to growth data
Within pairs, is the smallest twin also the one with the highest disease risk later in life? Since twins (or siblings) come from the same family, this test controls for any shared family effects, such as maternal nutrition, parental education or socio-economic status.
A second extension compares associations of differences in size at birth with differences in disease risk in monozygotic (MZ) and dizygotic (DZ) twin pairs. If associations of difference scores are larger in DZ than in MZ twin pairs, this is taken as evidence that the association is mediated by genetic factors.
These two methods can be considered as alternative approaches to the full bivariate analysis of MZ and DZ twin data. Using a bivariate structural equation model, the correlation between two traits can be decomposed into genetic and environmental correlations.
We address some statistical questions regarding the relation of difference scores (within MZ and DZ pairs) and genetic and environmental correlations. We show that the comparison of associations between MZ and DZ difference scores does not necessarily provide clear-cut answers to the question of how the relation of size at birth and later outcome is mediated.
We present an empirical application to data on stature, birth weight and height assessed in a large sample of Dutch adult MZ and DZ twin pairs. There is a significant association between size at birth (both weight and length) and later stature. Within MZ and DZ twin pairs, the largest/heaviest twin at birth is the one who is tallest later in life. Using a bivariate structural equation model, we show that the association of birth length/weight and adult stature is explained by shared genes as well as by correlated common family and unique environmental influences.
KeywordsBirth Weight Twin Pair Adult Stature Birth Length Birth Size
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