It’s a pretty even split. I do make sure that I have at least a passing interest in the topic, other than making money off it. But I don’t need to be an ‘expert’. And I don’t pick topics in health/nutrition/fitness, simply because it is too difficult to do really quick research for brief articles on those types of topics.

I mostly use content written by others and public domain works at first, but I also keep in mind that if a site really does well, I will want to expand on it, so I at least need to be interested enough to be willing to do research and write a fair amount on the topic if it comes to that.