I've been reading Sowell's Basic Economics (a fuller review in the future) but I was really struck by an argument he made about minimum wages.
He points out that a minimum wage actually makes it cheaper to discriminate. I've used the argument for a long time that price floors generally cause discrimination, but I've never heard it made this way.
Suppose you have two workers who can do $3 and $4 worth of work each hour, and you must employ at least one of them. With voluntary exchange and arbitrage they will end up getting paid $3 and $4, and the employer will by happy to hire one or both of them. However, if we institute a $5 minimum wage, the employer will discriminate and hire only the worker with the higher productivity. The employer is out a dollar, but they now have $2 to spend on capital to make their single employee more productive.