From my experience, it seems that +pop growth is the predominant factor. In one game I was fortunate enough to have an economic boom. My overall approval was a respectable 67% and my totat pop was 71.5B. I dropped taxes to 15% and approval rate soared to 100. I started counting the new tax payers every turn. I was gaining an even 2 billion new folks a turn to start, and 20 turns or so later, when the game ended (the ol' culture bomb, I love doing that), overall approval was down to about 72% but pop growth was still a steady 2 billion per turn. So, while I cannot prove it conclusively, my suspicion is that as long as morale is above the threshold point (at least 58) in an empire you are going to get the max pop growth accross the board your tech/wonders/empire traits allow.
Of course, to do this right one would have to look at growth rates of a single planet of a stable planet quality, stable +pop growth rate, and stable +morale bonus and see if and when the growth rate changes. Hmm, I might just do that next new game. Just sit around Sol without doing any expanding, exploring, researching, nothing that would change bonuses or morale to see when the slow down hits and how hard it is. In other words, is it steady fall off between 100 and the threshold or more of an all at once thing. I am guessing it is pretty rapid fall off once morale gets toward threshold.
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