Environmental contributions to preschoolers' semantic fluency.

Semantic fluency was examined in Hebrew-speaking 5-year-old monozygotic and dizygotic twins (N = 396, 198 pairs), 22% of them with mother-reported speech-related problems. There were positive correlations of similar magnitudes among monozygotic, same-sex dizygotic, and opposite-sex dizygotic twins. Analyses showed no genetic effects, alongside significant shared (39%) and non-shared environmental (61%) effects on fluency scores. The presence of speech-related problems in one twin affected the fluency score of the co-twin. A multivariate regression analysis revealed that parental education and length of stay at daycare significantly predicted fluency scores. We suggest that semantic fluency performance is highly affected by environmental factors at age 5 although genetic effects might emerge later on.