The most remarkable distinction across ethnic groups in the relative genetic and environmental influences on the two phenotypes was the absence of shared environmental influences on problem alcohol use in the AA subsample vs. the attribution of 33% of the variance in problem alcohol use to shared environment in the EA subsample. The best-fitting model for problem use in the EA subsample was more similar to models of age at first drink than models of severe problem drinking (i.e, AD) reported in previous studies, with shared environment playing a larger role than genetics. The heritability estimate of 21% is much lower than the range of 50–60% reported for AD models in the existing literature, which do not include any shared environmental effects. By contrast, the AA model more closely resembled AD models from the larger literature in its absence of shared environmental influences and its substantial (but slighter lower) heritability estimate of 40%. It was also more similar than the EA model to findings reported in the few prior twin studies examining problem use, which produced heritability estimates of 41%
