That's an interesting theorey. If it's correct that could explain it. But, I just checked the demographics in Massachusets in the 1800s and it shows that ther

That's an interesting theorey. If it's correct that could explain it. But, I just checked the demographics in Massachusets in the 1800s and it shows that there were 0 slaves. And it looks like minorities were in very very small numbers: https://en.wikipedia.org/wiki/1800_United_States_census https://en.wikipedia.org/wiki/1800_United_States_census I don't think the labor supply of oppressed minorities was influential enough to drive down the cost of labor. Yes, for railroads sure but not for construction of buildings.