Instrumental Variable Validity (The Effect, Videos on Causality, Ep 58)

Please visit www.theeffectbook.net to read The Effect online for free, or find links to purchase a physical copy or ebook.
The Effect is a book about research design and causal inference. How can we use data to learn about the world? How can we answer questions about whether X causes Y even if we can't run a randomized experiment? The book covers these things and plenty more. These videos are meant to accompany the book, although they can also be viewed on their own.
This video relates to material found in Chapter 19 of the book.
A version of this video without background music can be found here: • Instrumental Variable ...
One of the more important assumptions in instrumental variables is the assumption that our instrument is valid - that it's only related to the outcome via the treatment variable. What should we know about validity, and how can we think about whether it's true?