forms - Why might right aligned field labels be better?

For the longest time, I've used left-aligned labels in my forms, like this:

I did this because I thought having the labels left aligned made it easier for readers to scan the list of labels.

However, I found my axiom challenged recently while watching the video of Billy Hollis' talk from NDC 2011, in which Billy asserts that right-aligned labels are simply better, like this:

I've done a bunch of research and found a lot of discussion, including this UX question, which compares and contrasts the approaches, but nothing definitive.

In particular, I haven't been able to find anything to indicate why Billy Hollis changed his opinion from left- to right- alignment for labels (apparently he switched around 2007).

I figure Billy's too smart to have made the change without evidence - so I'm interested in seeing that evidence for myself.

Answer

Luke Wroblewski wrote about this in Top, Right or Left Aligned Form Labels (April, 2007).

In it, he references eyetracking data from an article by Matteo Penzo called Label Placement in Forms (July, 2006). Matteo drew several conclusions from this study, including that right-aligned labels have a lighter cognitive workload for users:

Alignment of labels—In most cases, when placing labels to the left of input fields, using left-aligned labels imposes a heavy cognitive workload on users. Placing labels above input fields is preferable, but if you choose to place them to the left of input fields, at least make them right aligned.

It's possible Billy saw Luke's article and changed his stance based on it.