Something is endogenous when you don’t know whether it’s a cause or an effect (or both). For example, lots of people note that people who go to college tend to make more money. But how much of this is because college boosts earning power, and how much is because smarter, harder-working, better-connected people tend to go to college in the first place? It’s endogenous.
.. you should ask “What about endogeneity?”
• Marginal versus average
• Present value and discounting
.. Present value means trying to figure out how much some long-term thing is worth today. To find present value, you have to use discounting, which means you have to decide how much less you value things that come far in the future. The more you want things right now, the higher your discount rate is, and the lower the present value of things like college degrees or business investments that take a long time to pay off.
• Conditional versus unconditional
.. If you lived in the Middle Ages and you made it to adulthood, you would probably live well past 35. While conditional life expectancy has increased since then, it hasn’t gone up by nearly as much as the unconditional version — reductions in infant mortality have been the biggest difference.
.. Individually, borrowing and spending money reduces your wealth. But in aggregate, debt doesn’t reduce the value of the whole world’s wealth, since one person’s debt is another person’s asset. When we consider our own lives, it makes sense to think from an individual perspective, but when we discuss government policy, it’s important to think in the aggregate.