Social networks affect many aspects of life, including the spread of diseases, the diffusion of information, the workers' productivity, and consumers' behavior. Little is known, however, about how these networks form and change. Estimating causal effects and mechanisms that drive social network formation and dynamics is challenging because of the complexity of engineering social relations in a controlled environment, endogeneity between network structure and individual characteristics, and the lack of timeresolved data about individuals' behavior. We leverage data from a sample of 1.5 million college students on Facebook, who wrote more than 630 million messages and 590 million posts over 4 years, to design a long-term natural experiment of friendship formation and social dynamics in the aftermath of a natural disaster. The analysis shows that affected individuals are more likely to strengthen interactions, while maintaining the same number of friends as unaffected individuals. Our findings suggest that the formation of social relationships may serve as a coping mechanism to deal with high-stress situations and build resilience in communities.social networks | natural disasters | causal inference | natural experiment | propensity score matching S ocial networks affect many aspects of life, including the spread of diseases (1), access to resources and information (2), the diffusion of knowledge (3, 4), productivity and stability of organizations (5, 6), and job prospects (7,8). In this paper, we conceptualize a natural experiment † by taking advantage of the well-defined local impact of a hurricane to gain a quantitative understanding of how these networks form and evolve. Our analysis provides insights into how to leverage social dynamics for affecting outcomes of interest, such as how to design policies that can aid rescue and recovery efforts or influence behavior and the economy.Establishing causal relationships in social network formation and dynamics has historically been difficult to study because of endogeneity between network structure and individual characteristics, and the cost of obtaining long time-series about individuals ' behavior (16, 17). In addition, large-scale randomized experiments are often not feasible because of the complexity of engineering social relations in a controlled environment, and the multitude of incentives that influence human behavior (18)(19)(20), and often because of privacy and IRB related issues (e.g., see refs. 21 and 22). Recent research tackle these challenges by developing behavioral models of network formation (23) that can support what-if analyses, and by using automated services such as Amazon Mechanical Turk (aws.amazon.com/documentation/ mturk) to carry out randomized human-subjects experiments of social dynamics in artificial environments, at scale (24-26). Related literature focuses on incentives, aiming at separating influence from selection effects, a task for which negative results exist in general (27, 28), using randomized experiments (29-32) and strategie...