Quote from kinggyppo:
Its hard to say what leads what, lots of debate. Both the spy and current month es are very liquid. Spy has a very tight spread .01 and very liquid options. But yes due its inherent liquidity its a good proxy for sp products. You may want to poke around et there are some threads on this. I should add that the institutions trade spx also known as the big due to its large tick size. If Lawrence Chan is around maybe he can give his opinion.