Whether or not a direct proportional tax would be optimal depends on whether or not the benefit one gets from having more money decreases in a directly proporti

Whether or not a direct proportional tax would be optimal depends on whether or not the benefit one gets from having more money decreases in a directly proportional way. I have no answers to this question (which should be answered in a statistical manner), but I fail to see a reason for believing a priori that a fixed percentage for everyone is the fairest form of taxation.