I don’t know of any studies showing superiority of a scrollbar on the right, but I do know one study that suggested the left is superior for most situations:
Kellener, E., Barnes, G.M. and
Lingard, R. (2001), Effects of scroll
bar orientation and item
justification, Proceedings of the
Human Factors and Ergonomics Society
45th Annual Meeting
The increase in clutter apparently is not an issue. The advantage comes from the scrollbar being nearer where the mouse pointer tends to be, and, thanks to a left-to-right layout of content, the mouse is more likely on the left than right (e.g., to select text or a control, like menu item). I know I find it painful to watch users alternate between the Back button in the upper left and the scrolling down in the lower right when searching multiple web pages, especially when they are using a laptop touchpad.
I don’t know why the scrollbar ended up on the right, but I can find only two cases where it was put on the left (NeXT’s OPENSTEP, and Oracle Applications User Interface Standards), so I guess the right seemed like the most natural place. Maybe the scrollbar is expected to be used after scanning the screen, so, like the OK and Cancel buttons, it’s placed where the eyes will find it then. In practice (shrug), it doesn’t seem to be that important. Users don’t seem to have a hard time finding a browser back button, after all.
Despite the advantages of a left scrollbar, however, I would advise against mixing apps with left and right scrollbars on the same workstation, at least, not without heavy testing to be sure it retains its advantages without causing problems on other apps. You don’t want users slewing the mouse one way then reversing because they forgot “where” they are. That could quickly wipe out any advantage for left scrollbars. Besides, with the advent of the mouse scroll wheel, the advantage of a left scrollbar has been diminished.