If I were to say that I’m thinking about having sex with my stepbrother, I guess you’d tell me to think again: sex with a sibling or even a stepsibling is just plain wrong – it’s not a morally acceptable action. The reason I’m posing this hypothetical proposition is because it’s worth considering why we find this kind of behaviour so wrong. Is this judgment based on a rationally derived principle about maximising good and minimising harm? Surely sex with my sibling would harm our relationship, not to mention the rest of our family’s relationship with each of us. Or is the moral judgment here based simply on the fact that sibling sex makes us more than a little queasy? In other words, are our moral beliefs merely gut feelings – quite literally stemming from our body’s tendency to become repulsed by certain human behaviours?