I'm tapping out of this conversation. It's predisposing me towards racism. I'm sure anybody actually interested will have no problem finding a book on game theory.

In countries without mandatory voting, if voting is more inconvenient for certain groups than for others, the latter will be over-weighed in the election. With mandatory voting, casting a valid vote is no more and no less inconvenient than spoiling the ballot, so that's not an issue -- all eligible people who ceteris paribus would prefer, no matter how slightly, to vote will do so.

(Unlike wedrifid I'm not in a country with mandatory voting, BTW.)