My theory is that it is because of the margin, which is pretty much always a straight line. We are more accustomed to there being wider space at the end of a li

My theory is that it is because of the margin, which is pretty much always a straight line. We are more accustomed to there being wider space at the end of a line (except in justified text, which is more common in print than on screens where a wall of text can look pretty obnoxious.) Geography aside, the scroll is on the same side that you thumb to change pages in a book. Putting a scroll bar on the right doesn't make us feel like the margin is in the wrong place, or that you're changing "pages" from the wrong side of the screen or window. Having said that, are Arabic scroll bars on the left? I've worked in SA, but I never noticed since I used my own laptop. I imagine the sideways scrollbar would be really counterintuitive on the top vs the bottom.