.....Human morality shares with language that it is far too complex to be learned through trial and error, and far too variable to be genetically programmed. Some cultures permit the killing of newborns, whereas others debate abortion of the unborn. Some cultures disapprove of premarital sex, whereas others encourage it as part of a healthy sexual education. The gravest error of biologists speculating about the origin of morality has been to ignore this variability, and to downplay the learned character of ethical principles.

Possibly we are born, not with any specific social norms, but with a learning agenda that tells us which information to imbibe and how to organize it. We could then figure out, understand, and eventually internalize the moral fabric of our native society. Because a similar learning agenda seems to regulate language acquisition, I will speak of moral ability as a parallel to language ability. In a sense, we are imprinted upon a particular moral system through a process that, though hundreds of times more complicated than the imprinting of birds, may be just as effective and lasting. And, as in birds, the outcome may deviate from the norm. A friend of mine attributes the thrill she gets from smuggling to the praise she received as a young girl whenever she managed to stash away food (during World War II she spent years in a Japanese concentration camp). The criminal justice system of course encounters far more serious deviations, and these too can often be traced to lessons received, or not received, during sensitive phases of moral development.

Does this make morality a biological or a cultural phenomenon? There really is no simple answer to this sort of question, which has been compared to asking whether percussive sounds are produced by drummers or by drums. If we have learned anything from the debate, it is that nature and nurture can be only partially disentangled. The same is true of relatively simple processes, such as the effect of light on plants. It a plant in a sunny spot grows taller than one in the shade, it is not because of either genetics or the environment, but both....Environmental influences - including human culture - vary with genetic "substrate" on which they act....