What ultimate causes explain the rise of democracies and the decline of slavery?

My bro asked me a thought-provoking question recently- essentially, what ultimate causes explain the rise of democracies and the decline of slavery (at least through much of the Western world)?

I've discussed the subject with some friends in the Mormon Transhumanist Association, and though I'm no Jared Diamond, here goes.

1. Printing Tech. The ability to quickly and broadly spread stories and ideas (e.g. the fictional Uncle Tom's Cabin or Locke's writings) created a sociopolitical environment capable of more rapid change than was feasible before. Other factors account for the direction of that change.
2. Mechanical Tech Advances. Significant inventions altered the allocation (and mechanization) of labor more broadly and more rapidly than before, creating a ripe milieu for labor change. (Think mechanized loom, steel plough, combine, reaper, and steam engines). The resultant increases in productivity altered the historical power balance between labor producers and consumers in many industries, opening up empowerment of the labor force and leading toward democratization and abolition. 
3. The rise of certain ideas. Niall Ferguson, in Civilization: The West and the Rest suggests six. Competition, science, the rule of law, modern medicine, consumerism, and the work ethic. 

I'm still noodling on this subject- Trye, I invite you to respond and continue the conversation. It's at times like these that I wish I had a relaxed week-long camping trip with Yuval Harari, Jared Diamond, and Bill Bryson to gnaw on the subject out loud together. :-)

(Btw- any MTAers that want specific attribution, let me know and I'll reference you).