Heritability of neurocognitive functioning in the elderly: Evidence from an Italian twin study

BACKGROUND: the genetic and environmental origins of individual differences in specific cognitive abilities in the elderly are poorly understood. One reason is the lack of studies performed in cohorts with normal cognitive functions. OBJECTIVE: to estimate the relative contributions of genetic and environmental factors in determining inter-individual variation in neurocognitive abilities in the Italian population. DESIGN: cross-sectional analysis of twin data. SETTING: a sample of older twins with normal cognition from the population-based Italian Twin Registry (ITR). SUBJECTS: twin pairs resident in Rome and born between 1926 and 1940, identified through the ITR in 2002. The final study population included 93 twin pairs. METHODS: subjects underwent neuropsychological tests providing information about different cognitive domains. The contributions of genetic and environmental effects were assessed using standard univariate twin modelling based on linear structural equations. RESULTS: the best-fitting model incorporated additive genetic (A) and unique environmental (E) sources of variance for the following tests: Mini-Mental State Examination (A = 55%), Raven (A = 56%), Attentional Matrices (A = 79%), Copying Drawings (A = 69%) and Story Recall (A = 54%). For Phonological and Semantic Verbal Fluency, the best model included non-additive (D) and unique environmental influences (D = 62 and 54%, respectively). Cigarette smoking was estimated to be negatively associated with the score of Phonological Verbal Fluency. For Token test, the inter-individual variance was entirely due to environmental factors not shared by the twins. CONCLUSION: our data showed that most of the specific cognitive abilities are moderately to highly heritable, and that the environmental factors of relevance for these abilities are those causing within-family differences.