infrastructure, hospitals, roads, schools. you have to build more houses. otherwise the congestion costs become significant. we're not very good at doing any of those things. >> what do you make of the arts made in the united states where there are economists and scholars who say when you look at the effect of migration from mexico on low income americans, it's not clear it's beneficial. it probably hurts them. these are the same people who compete for the same low wage. you have to build this capacity. are they right? is donald trump right? >> some of the propositions he's putting forward seem to be sort of basically mad. if you have a large number of people in the country you can't throw them out. economists are debating these questions. they are really big arguments