The problem is that ethics is very complicated – it's really hard to formulate a complete moral system that is internally consistent but which doesn't lead to counterintuitive results. However, if the basic ideas underlying a particular theory seem obviously correct to some, then they might come to accept its conclusions no matter how counterintuitive. Thus, Kant was famously led to the view that it's wrong to lie even to an ax-wielding murderer who inquires you on the whereabouts of your children. And just the other day, some utilitarian fans of Sam Harris told me they saw nothing wrong with the idea of a surgeon killing one healthy patient in order to harvest his organs and save five others.
Sam Harris himself, like most utilitarians faced with this well-known objection to their theory, expressed his disapproval of such a killing by appealing to the standard utilitarian reply – namely, that such an organ-harvesting policy would have bad consequences in the long run, as it would make people avoid doctors. But the problem with this reply is that it doesn't address what is really wrong with the act. The wrongness of intentionally killing an innocent person to save five others has almost nothing to do with the consequences for society in general – it has to do with the wrongness of killing an innocent person! And so the standard utilitarian reply fails when we tweak the example just a little – for instance, by making it about a single instance in which a surgeon successfully makes the death of the innocent person look like an accident.
When faced with this new version of the problem, a utilitarian might very well bite the bullet and conclude that, contrary to one's initial intuitions, the act is not wrong at all. And that is exactly what those Sam Harris fans did.
But in addition, these people, having read The Moral Landscape, were convinced their view was backed up by science, and was therefore factually correct. And that made me wonder if the belief that one's preferred moral system – whether utilitarian, Kantian, biblical, or what have you – is true makes one all the more prone to accepting its counterintuitve conclusions. It seems very likely, and if so, then this is one area where a subjectivist approach to ethics appears preferable. If one doesn't believe a moral theory is factually correct, then one might be more open to changing one's mind about it.