In Chapter 2, I employ the educational production function to identify the different effects of making a friend of the same gender and the opposite gender in a school network. Unlike other gender peer effects literature that only quantifies the causal effects of the proportion of girls in an aggregated level, such as other students in the same class, grade, or dorm, I study the gender of the five best friends nominated by the student. I address the endogeneity of friendship composition by employing a novel set of instrumental variables for the number of same-gender and opposite-gender friends. We find that having more friends, especially in the early accumulation stage, lowers the test scores. We also explore the mechanisms. In Chapter 3, I investigate the role of social learning in enrollment decisions for a public pension scheme. All else equal, if a qualified rural resident moves from a community where no other co- villagers participate in the new pension scheme to a community that is fully covered by the pension scheme, the probability of an individual enrolling by 0.541 percentage point. We use robustness checks to illustrate that the estimated peer effects are not driven by the common unobserved factors, but by social interactions. In Chapter 4, we use the survey data on Chinese middle students and the instrumental variables method to explore the different effects of making friends with the same gender and the opposite gender in a school network on mental health. The empirical results find that having a larger number of same-gender friends improves mental health but having a larger number of opposite-gender friends hurts mental health. / Doctor of Philosophy / We need human connections. Along with other assets, such as money and skills, networks and relationships are resources that could help with economic outcomes in our daily lives. The rapid development of the Internet and the intelligentization of digital devices such as mobile phones have made it easier to establish relationships with others. They also generate much more data nowadays that makes it possible to study social relationships. In this dissertation, we mainly discuss two aspects of social networks. First, we use popularity as a measure of social capital and study how social capital influence middle school students' academic outcomes and mental health outcomes using Chinese data. Given that middle school students are in the embryonic stage of personal emotional development, we distinguish friends by of the same gender or not. We find that popularity with the same gender and the opposite gender differently impacts the outcomes. Second, it is intuitive that under the influence or pressure of a group, an individual tends to make his or her speech and behavior consistent with the group. Therefore, we are interested in if an individual's choice will be driven by other people's choices in the same group. We consider the adoption of a newly introduced pension program for rural residents in China. Besides demographic characteristics, a person's decision is also influenced by those around them. If a higher proportion of his or her co-villagers choose to join the pension plan, he or she is more likely to join.