Put as simply as possible, the thicker one's accent, as perceived by the listener, the less likely one is to be believed. Although obviously a problem for newcomers to Canada, it would be a barrier for those of us trying to support them too. For more detail see There's Something About Johnny Foreigner at the Mind Hacks blog.
As mentioned in the Mind Hacks article this finding appears to relate to a well-studied psychological phenomenon termed cognitive fluency. I've mentioned it here a couple of times before: