6 mo dental visits

I'm sorry of this sounds like a troll - it isn't.

When I was a kid the dentist wanted to see us once a year.
Now they all seem to say to come in every 6 months. Is this
recommendation based on some studies, clinical observations,
just a vogue, or what?

Similar question for the x-rays - are there studies
suggesting an optimal frequency (or if they add anything to
the exam at all?

And about that mercury in the amalgams...... (just kidding)

Thanks in advance

--
00doc