Genetic and environmental molarity and modularity of cognitive functioning in 2-year-old twins

We investigated genetic and environmental influences on the similarity and differences among five tests of cognitive abilities in 1958 pairs of same-sex 2-year-old twins born in the UK in 1994. Using a common pathway model, genetic and shared environmental factors contributed to the overlap (molarity) among cognitive abilities, as well as to the discrepancies (modularity) among cognitive abilities, These results suggest a developmental trend from modularity to molarity when considered in relation to multivariate genetic results later in life that show that genetic effects on cognitive abilities contribute primarily to molarity rather than modularity. (C) 2001 Elsevier Science Inc. All rights reserved.