The estimation of 6 or fewer objects (‘subitizing’) is considered to be a form of judgment which differs in speed and accuracy from the ‘estimation’ of 7 objects or more. In this study, 120 complete families (father, mother and one or more children), and a further 100 families lacking either father or mother, were asked to estimate the numbers of marbles or balls shown to them in transparent bags. The results indicate that there exist at least two phenotypes of judgment of numerousness: 1. subitizing of 6 marbles and 5 ping-pong balls, with high heritabilities; 2. estimation of 34 small, 15 small and 15 large marbles, with zero to low heritabilities, in the same population tested at the same time. The heritability estimates are based on parent: offspring correlations, corrected for assortative mating. Assortative mating was found for the 5 and 6 objects only. Father : son, mother : son, father : daughter, mother : daughter correlations were almost identical and gave no indication of differential maternal or paternal influences on estimation. Analyses of phenotypic distribution of offspring from different types of matings, based on under, over, and correct estimation, indicate that estimating two sets of less than 7 objects (subitizing) may be inherited on a polygenic basis while estimating sets of 15 or 34 objects shows low to no genotypic control.

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