Logic behind twin research

The logic behind twin research. The scatter plots depict how much the two types of (reared-together) twins resemble their co-twin on reading ability. Each dot represents the reading scores of both children within a pair. It can be seen that monozygotic twins are much more alike. From this it can be concluded that differences between children are largely due to genetic differences. The data come from van Bergen et al. (2018) and represent word-reading fluency test scores in Grade 2 of twin pairs with complete data. The score is the number of words read correctly within 1 minute. In this sample the monozygotic and dizygotic twin correlations were .84 and .46 respectively, which yield estimates using the Falconer formulas (Falconer, 1960) of A = .76, C = .08, and E = .16