When I was an undergrad, a group of historians were publishing new studies of slavery, based on statistical analysis that suggested that slavery "wasn't so bad" because relatively few slaves were actually beaten or killed in the course of regular plantation life. They found this was so because the slaveowners didn't want to endanger their investment and damage their property. There were no statistics available on the psychological effects of being property, so this detail was not included in the studies.

In my opinion, it wouldn't matter if the slaves had been allowed to live in mansions and were treated like solid gold. People as property is wrong, and I would bet most of them would have traded it in a second for a hole-filled shack on their own piece of land as free people. Some people have also pointed out that the descendants of slaves live better than they would have if their forefathers has remained in Africa. That doesn't make it okay either.

I'm sure you came to similar conclusions in response to that study, and while I have not seen "statistics" on the psychological effects, there is oral and written history aplenty testifying to it.

A lion in a zoo usually lives a long, healthy life, with good food, mates, and the best medical care. It's life is less harsh than in the wild, with less danger, and more security. But it isn't really a lion any more.

Obligatory political note:

Oddly, when I draw a similar "birds in a gilded cage" analogy to the welfare state, which provides "citizens" their every need in return for political control of the citizen, people look at me like I'm talking in Swahili.