I had always heard that since most people are right handed they just naturally gravitate to the right when given a choice. My son did his science project on that premise: he observed traffic patterns for loading at several rides in Six Flags over the course of a day, and statistically the left lane was always faster (i.e. had less people in it at any given time). However, there were abberations as mentioned above (a huge group walking in at one time, line hoppers going from the right to left side midway if the line "looked better" etc). From personally experience, I can tell you the slowest line is ALWAYS the one I am in, whether it is at WDW or the grocery store!