Should developers stick to what they know?

Not just in gaming, but in life in general it's important to take risks, to try something you're not sure your good at, but as video games have shown us, this isn't always the most wise thing to do. The question I pose is this; should video game developers create games for genres which they are experienced with working in, or should they try to branch out to try something new?

I can't say that I sway either way – I don't know which side of the argument I support more, but lets take a look at the positives and negatives of developers sticking to what they know and to what they don't.