First, and most importantly, I am very grateful to my advisor, Edward Lazear, who first introduced me to the field of labor economics at Stanford; special thanks also go to the late Gary Becker, who encouraged me to work on this topic during my postdoctoral study under his guidance at the University of Chicago. I benefited immensely from the collaboration with my fellow researchers, Wenzheng Huang and Hui Wang, who co-authored many articles with me on this topic. I am grateful to my research and executive assistants, ZhengYu Fang, Xiaomeng Xu, and Xinxin Wang, who helped with data gathering and analysis, and also to my friends, Wei Gu and Ted Fishman, who gave thoughtful feedback throughout the process.