I just read a forthcoming paper (in Mind and Language) by Shaun Nichols and several others, which argues that it is rational to develop moral rules that distinguish (for example) acts and omissions. The relevant idea of "rational" is from rational concept formation.
When you learn a new concept, it is best not to generalize it too much. In some experiments, subjects were given examples of rule violations, for learning, and tested with other examples. When the learning examples were of the form "X did an action A that caused outcome C to happen", subjects generalized this to similar test examples with other examples of A and C. But they did not consider examples of the form "X failed to do B, which would have prevented C from happening" to be violations of the rule. In order to teach subjects that the rule applied to omissions as well as acts, the training had to include omission cases as examples of rule violations.
This behavior of subjects makes perfect sense in the case of arbitrary rules, and even legal rules. But I was bothered because I don't think moral rules should be arbitrary in this way.
One possible explanation of the difference is that sophisticated moral rules arise from reflection on the social rules that we have learned. Specifically, we reflect by asking questions about purposes (which I call "search for goals" in some places). When we see an example of a rule and ask about its purpose, we might discover what general purpose it serves. We can then think about how to generalize it so that it serves that purpose. If it does not serve the purpose in some cases, or if it could serve the same purpose better by a modification, then we can think about improving it.
The same process is part of what it means to understand a design such as a mathematical formula, according to my interpretation of David Perkins' book "Knowledge as Design". For example, we understand the formula for the area of a parallelogram (and its associated arguments) by finding that the argument for this rule serves the purpose of converting the parallelogram to a rectangle, and we already know how to find the area of a rectangle. Once we discover this connection, we can apply the same principle elsewhere, as Max Wertheimer shows in the first chapter of "Productive thinking". We can transfer the principle to cases where it applies while avoiding transfer to other cases.
Similarly, a law with a "loophole" is an example of a rule that is crafted in a way that fails to serve its purpose. We can fix laws by removing loopholes.
A law that gives rights, such as the right to vote, drive, or own property, to men but not to women does not seem to serve reasonable accounts of the purposes of such rights-granting laws. We have trouble coming up with a purpose that applies to men but not women. Any such purpose seems arbitrary; it could just as well distinguish people with odd and even birthdays. Such a search for purposes is, I think, the sort of reflection that Peter Singer discussed in "The expanding circle".
Thus it is one thing to learn a rule, but it is another to understand the rule in a way that allows us to ask whether it serves its purpose as well as it could, and, if not, what could replace it. It may be rational from the perspective of learning to learn whatever we are taught about what to do and not do, but, if this is all we did, we would cut off the possibility of improving these rules.
Could most deontological rules survive this kind of questioning?