Slavery: Roman Empire

Something I feel is often overlooked about the institution of slavery during the Roman Empire, is that it worked as a social filter. More recent slavery failed as it worked the opposite way, keeping people in slavery generation after generation.

What do I mean by social filter? The institution funnelled people from the outside, into becoming civilised. Now I'm grossly exaggerating of course, see a Wikipedia article below for a more detailed description. However I think my generalisation still holds.

I use an example, the instance is Julius Caesar's Gallic Wars. When a region is conquered he takes a certain number as slaves. Wholesalers followed the army and they would take the slaves and sell them. This would have had two effects, the first being the conquered get to rid themselves of their troublemakers and underclass (that's because they had a say in who got enslaved). It also kept everyone left that wasn't enslaved behaving nicely.

Then the slaves once taken supported the Roman economy by conducting mostly menial tasks (though some were highly skilled and others were treated brutally in the mines, they are the exception not the rule) and they then had the chance to become free and a Roman citizen of low rank. This wasn't just a rare occurrence, laws had to be passed stopping the rapid freeing of slaves, it became so commonplace. And slaves were generally looked after with laws evolving to protect them from mistreatment. Some were expelled from Rome for being too dangerous, but most led productive lives after being freed, including those expelled.

The social filter remained effective for longer than the period of enslavement as freed slaves were connected to their former owners who became the freed slaves patron. They had obligations to one another. And critically, the children of freed slaves became Roman citizens without restriction. 

For a good intro to Roman slavery see the following Wikipedia article: https://en.wikipedia.org/wiki/Slavery_in_ancient_Rome

And a plug for my novel Dana Point, which has the Herculean society biochemically ranked. Status is determined by blood test at birth but individuals can rise by achievement. If they commit a crime they're busted back to the rank they were born with. It's the way I see a fully advanced Roman society evolving if it was allowed to.