"A related question is how well Haiti can do as an anarchistic society. Haiti is one right now and arguably many parts of the Haitian countryside have been quasi-anarchistic for a long time, ruled by either custom or gangs. ... It's evidence that the Haitian social fabric is a lot stronger than many people thought."
I suspect a belief in voodoo lessens criminal predation in situations without effective policing (which is most of the time in Haiti). If there is no law-and-order, what is to stop you from doing bad things to other people? Well, beyond payback from lynch law, family vendetta, and mafia vengeance in this world, there is the threat of your victim or victim's surviving relatives putting a curse on you in the spirit world.
Anthropologist Henry Harpending, who spent 42 months living in Africa and liked it so much that he seriously considered leaving academia to become a safari hunting guide, has said that modernity ruins morals in tribal villages in Africa. Isolated villages have a stable culture underpinned by fear of retribution by black magic. It's not a culture conducive to progress, but it's at least a culture adapted to the local conditions, such as they are. Once a road comes to town, however, and people stop fearing quite so strongly that if they do something bad to a neighbor, they'll suffer vengeance from the spirit world, things fall apart. People become more likely to do something bad to their neighbors.
Of course, voodoo has its disadvantages: it has no ethical content. Deities do whatever they feel like, and the more outrageous the bribe (e.g., human sacrifice in Africa), the more they might feel like helping you and hurting your enemies. Papa Doc Duvalier, a superbly educated doctor and intellectual, studied his patients' beliefs, and used them to position himself as a voodoo sorcerer whom you had better vote for and obey, if you knew what was good for you.
By the way, the fad for changing the spelling of "voodoo" to "vodou" in news stories about Haiti is just another example of the long-running campaign to make the American public more ignorant by cutting them off from their past learning by changing names. The intention is to make Americans' eyes glaze over when they see the word "vodou" instead of light up when they see "voodoo."
We'll know that liberals are sincere when they start referring to tax-cutting as "vodou economics."
Don't count on it.