 The ACE model is a factor analytic model that partitions phenotypic variance (represented with boxes) into three latent components (represented with circles). The first latent factor, ‘A’ represents ‘additive’ genetic variance and captures genetic similarities and dissimilarities between twin pairs, as monozygotic (MZ) ‘identical’ twins share roughly 100% of their DNA while dizygotic (DZ) ‘fraternal’ twins share roughly 50% of their DNA, just like normal siblings. The component ‘C’ represents ‘common’ environment, signifying environmental variance that is shared by twin pairs reared together. For example, because both MZ and DZ twin pairs live in the same home, the correlation between their shared environment equals 1. Finally, ‘E’ represents ‘unique’ or non-shared environmental variance, capturing environmental variables that vary between twin pairs. In effect, it is the environmental variation that is not shared by twins outside the home which ends up getting couched as ‘E’, as is the case when twins attend different institutions or undergo different life events. 