Forthcoming as book chapter of
The Handbook of US Consumer Economics
Edited by Benjamin Mandel and Andrew Haughwout
 We gratefully thank Dr. Benjamin Mandel for providing the great opportunity, his warm help, and endless patience through this project. Miaojie Yu thanks China’s National Science Foundation (No. 71625007, and 71573006) and China’s Social Science Foundation (No.16AZD003) for their financial supports. All errors are ours.