Let's look at the context surrounding the sentence you referenced.
A rich line of behavioral genetic research has analyzed samples of kinship pairs (e.g., twins) to estimate the proportion of variance in antisocial phenotypes that is due to genetic influences. Results of these studies, which have been based on thousands of sibling pairs, have pointed to the inescapable conclusion that genetic factors are implicated–at least to some degree–in the etiology of violence. Precisely how influential genetic factors are, however, is difficult to garner when examining single studies because heritability estimates wax and wane from study to study based on sample characteristics and methodological differences. A number of meta-analyses (Ferguson, in press; Mason & Frick, 1994; Miles & Carey, 1997; Rhee & Waldman, 2002) and literature reviews (Moffitt, 2005) have thus been conducted as a way of summarizing the findings from these extant behavioral genetic studies. Overall, the conclusions reached by these studies have been highly consistent in showing that approximately 50% of the variance in antisocial phenotypes is the result of genetic factors.
So as you said, he's basically saying that genetic factors influence antisocial behavior. He's saying that 50% of the deviation from what would be expected based on environmental and social factors can be accounted for by genetic hereditary traits.