Behavioral genetic investigations have consistently demonstrated large genetic influences for the core symptom dimensions of attention-deficit/hyperactivity disorder (ADHD), namely inattention (INATT) and hyperactivity (HYP). Yet little is known regarding potential similarities and differences in the type of genetic influence (i.e., additive vs. nonadditive) on INATT and HYP. As these symptom dimensions form the basis of the current Diagnostic and Statistical Manual of Mental Disorders subtype classification system, evidence of differential genetic influences would have important implications for research investigating causal mechanisms for ADHD. The current meta-analysis aimed to investigate the nature of etiological influences for INATT and HYP by comparing the type and magnitude of genetic and environmental influences each. A comprehensive literature search yielded 79 twin and adoption studies of INATT and/or HYP. Of these, 13 samples of INATT and 9 samples of HYP were retained for analysis. Results indicated that both dimensions were highly heritable (genetic factors accounted for 71% and 73% of the variance in INATT and HYP, respectively). However, the 2 dimensions were distinct as to the type of genetic influence. Dominant genetic effects were significantly larger for INATT than for HYP, whereas additive genetic effects were larger for HYP than for INATT. Estimates of unique environmental effects were small to moderate and shared environmental effects were negligible for both symptom dimensions. The pattern of results generally persisted across several moderating factors, including gender, age, informant, and measurement method. These findings highlight the need for future studies to disambiguate INATT and HYP when investigating the causal mechanisms, and particularly genetic influences, behind ADHD.