Cognitive neuroscientists have anticipated the union of neural and behavioral science with ethics (Gazzaniga 2005). The identification of an ethical rule—the dictum that we should treat others in the manner in which we would like to be treated—apparently widespread among human societies suggests a dependence on fundamental human brain mechanisms. Now, studies of neural and molecular mechanisms that underlie the feeling of fear suggest how this form of ethical behavior is produced. Counterintuitively, a new theory presented here states that it (...) is actually a loss of social information that leads to sharing others' fears with our own, thus allowing us to treat others as we would like to be treated. Adding to that hypothetical mechanism is the well-studied predilection toward affiliative behaviors. Thus, even as Chomsky hypothesizes that humans are predisposed to utter grammatical sentences, we propose that humans are 'wired for reciprocity'. However, these two neural forces supporting ethical behavior do not explain individual or collective violence. At any given moment, the ability to produce behavior that obeys this ethical rule is proposed to depend on a balance between mechanisms for prosocial and antisocial behaviors. That balance results not only from genetic influences on temperament but also from environmental effects particularly during critical neonatal and pubertal periods. (shrink)