malum in se (something is wrong for its own sake) and only malum prohibitum (something is wrong because there is a law prohibiting it). An interesting question for malum in se is what makes something wrong for its own sake? Interesting research with small children sheds light on the mental origins of the distinction. From the Wall Street Journal's "Zazes, Flurps and the Moral World of Kids":
Back in the 1980s, Judith Smetana and colleagues discovered that very young kids could discriminate between genuinely moral principles and mere social conventions. First, the researchers asked about everyday rules—a rule that you can't be mean to other children, for instance, or that you have to hang up your clothes. The children said that, of course, breaking the rules was wrong. But then the researchers asked another question: What would you think if teachers and parents changed the rules to say that being mean and dropping clothes were OK?
Children as young as 2 said that, in that case, it would be OK to drop your clothes, but not to be mean. No matter what the authorities decreed, hurting others, even just hurting their feelings, was always wrong. It's a strikingly robust result—true for children from Brazil to Korea. Poignantly, even abused children thought that hurting other people was intrinsically wrong.
This might leave you feeling more cheerful about human nature. But in the new study, Dr. Rhodes asked similar moral questions about the Zazes and Flurps. The 4-year-olds said it would always be wrong for Zazes to hurt the feelings of others in their group. But if teachers decided that Zazes could hurt Flurps' feelings, then it would be OK to do so. Intrinsic moral obligations only extended to members of their own group.
The 4-year-olds demonstrate the deep roots of an ethical tension that has divided philosophers for centuries. We feel that our moral principles should be universal, but we simultaneously feel that there is something special about our obligations to our own group, whether it's a family, clan or country.
So even though there are moral origins to the distinction between malum in se and malum prohibitum acts, those moral principles underlying the distinction are not universal -- and not really that "moral" either, to the extent that they justify otherwise wrongful actions against people who happen to be different enough to somehow justify mistreatment.