I disagree with the idea that modal realism, whether right or not, changes the chances of any particular hypothesis like that being true. I am not saying that we can never have a rational belief about whether or not modal realism is true: There may or may not be a philosophical justification for modal realism. However, I do think that whether modal realism applies has no bearing on the probability of you being in some situation, such as in a computer simulation. I think this issue needs debating, so for that purpose I have asserted this is a rule, which I call "The Principle of Modal Realism Equivalence", and that gives us something well-defined to argue for or against. I define and assert the rule, and give a (short) justification of it here: http://www.paul-almond.com/ModalRealismEquivalence.pdf.