Why do modern GUIs tend to have buttons on the side of message boxes?

Question

Message boxes used to have buttons centered horizontally. E.g. here's one you'd get with MessageBoxW API function on Windows XP:

But more modern systems have message boxes with the buttons on the side, like this same message box opened on Windows 7:

Similar results can be seen with Qt3 vs Qt4+:

Similar pattern holds for message boxes with more than one button. See for example the following "Save changes?" prompts from Notepad on Windows XP and Windows 7:

The message boxes with centered buttons look nicer due to symmetry, so there must have been some good reason for the change.

What is the reason for this switch from centering the buttons to side-justifying them? Do buttons on the side have some important advantage over those in the middle?

Answer 1

Ruslan, if you look at older — dare I say, "historical?" ;) — screenshots, you can see that button placement differs considerably between OS platforms.

The Guidebook GUI Gallery website is an awesome resource to compare such details.

I haven't seen any research yet on the impact of left/center/right alignment of buttons in dialogs, but the general rule of striving for consistency applies to button placement as well, of course.

Placing key widgets in the same relative location in every context makes it easier for users to find them, and it reduces error rate. In particular, this applies to the key Cancel/Confirmation button pair.

Both macOS and Windows have defined simple principles on where to place those buttons, and which order they need to be in: you'll typically find those buttons in the botton right corner of a dialog on both OSs. This convention dates back to Mac OS 7 from 1991 and to Window 95/NT4 from 1995/1996. (As seen, e.g., in Open File dialogs or Time and Date Settings.)

So I wonder: have you observed changing button placements for dialogs (or, rather, alerts) that contain more than one button, Ruslan? If not, maybe it was a purely aesthetic consideration for that particular one-button case?