e., a tendency to engage in "antisocial" behaviors such as behavior misconduct, substance abuse, and other external behaviors, in such families. In other words, in many of the twins, when one twin exhibited antisocial behavior, so did the other (Hick, Krueger, Iacono, McGue & Patrick 1).
The study also found an increased likelihood for the twins to exhibit similar specific external behaviors, such as alcoholism or drug addiction. That is, where both twins exhibited antisocial behaviors, they also tended to exhibit the same antisocial behaviors. Unable to establish a genetic link, however, the study concluded, "environmental factors such as peer pressure may influence the way in which a genetic vulnerability to externalizing behaviors is expressed" (Hick, Krueger, Iacono, McGue & Patrick 1). Thus, the study concluded that while there may be a genetically transmitted vulnerability to externalizing antisocial behaviors, the manner in which this vulnerability is expressed - the particular antisocial behaviors expressed - must be the result of environmental factors. Thus, according to this study, while Kyle and Stan might share a genetic tendency to act antisocially, whether and how they act so is likely based on environmental factors.
Nonetheless, critics argue that there is, in fact, no way to identify specifically a genetic trait that controls external behaviors such as thes