Twin study of verbal and spatial abilities

Verbal and spatial abilities are recognized as highly heritable specific cognitive abilities. The purpose of this study was to perform the first behavioral genetic analysis of these abilities in a Croatian sample. A total of 160 same-sex twin pairs took part in the investigation. Their ages ranged from 15 to 19 years. Pairs with doubtful zygosity were excluded from the analysis, so the final analysis was carried out on 71 monozygotic (35 male and 36 female) and 78 dizygotic (43 male and 35 female) twin pairs. The twins were tested in small groups with four tests designed to measure two factors of verbal abilities (vocabulary and word fluency) and two factors of spatial abilities (spatial visualization and spatial orientation). For each of the tests substantial heritable variation was found. Respectively, intraclass correlations for monozygotic and dizygotic twins were 0.75 and 0.44 for visualization, 0.58 and 0.33 for spatial orientation, 0.67 and 0.41 for word fluency, and 0.74 and 0.41 for vocabulary. In addition, data were compared with several univariate LISREL models. Univariate model fitting resulted in heritability estimates of 63, 49, 52 and 61% for visualization, spatial orientation, word fluency and vocabulary, respectively.