You get +20% research bonus for every prerequisite you have, required and optional. There was (is?) an exploit to that, not sure if it had been addressed in Warlords.
Suppose that you have researched Preisthood and AH. Does that mean you have a 40% research bonus on Writing? And if, in addition, you have Pottery, does this meam the Writing reserch bonus is 60%?
Just wondering if the bonuses accumulate in this way.