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It may be that an ability to kill other human beings confers a reproductive advantage, as David Buss argues, ("Hardwired to kill", May 13), though a life spent behind bars does reduce opportunities for procreation. In that case, natural selection would favour the ability to kill other humans, provided that it were genetically determined to any extent.

It is by no means obvious that it is. After all, the ability to drive a car or to win the lottery gives some reproductive advantage but is probably not inheritable.

What would raise this speculation to a scientific theory is some empirical evidence, in the form of showing that killers tend to be more prolific and also that the descendants of killers are more likely than the average also to be killers, even when non-genetic factors are filtered out.

Only when he has such evidence is Buss competent to be an expert witness.

As for the moral significance of this theory, if it were established, it would confirm only that not everything natural is ethical, which is already quite obvious. Indeed, ethics might be understood as a countervailing force against what comes naturally.

Anthony Matthew. Leicester