"Both exogenous and endogenous. ...[H]uman societies have changed dramatically over the past couple of hundred years, and you have to take that into account. ...Many people talk about now, especially because of , the effect of technology, the automation, and of robotization, and that is certainly a force that reduces labor supply. But other things... played in the United States at the level of labor supply... first... the baby boom that created a large cohort of workers; and secondly the... massive entry of women into the labor force; and immigration. Immigration actually is much discussed but numerically it's... slightly less important than the... other demographic forces that we're talking about. ...What's exogenous, what's endogenous? It's really a matter of what our best model is, because you can endogenize things. But some things cannot be endogenized, so there are automation processes... this is a very long term process that has been happening over thousands of years... And so in my model that's clearly an exogenous mechanism. But what's the most important (to me) endogenous mechanism is the last thing that I included in the model, which is the attitudes... Think about it as institutions. ...Labor promoting institutions were installed in the United States as a result of the New Deal, and they worked very well until [the] late 70s, and then they... started to be dismantled... especially under the Reagan administration. And so I use the mininum wage as a proxy for the elite attitudes towards workers. It seems to work quite well, but we could use other proxies such as illegal anti-labor moves by firms. So that is an endogenous mechanism in theory because... essentially, ...when ...[a] crisis ...either destroys part of the elites, or frightens them so much that they install institutions that are more pro-labor, and that lasts until the collective memory of the crisis fades, and then you have... [a] recycling process... in my book I unpack these ideas."