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The idea that skills, technology, and knowledge, are spatially concentrated, has a long academic tradition. Yet, only recently this hypothesis has been empirically formalized and corroborated at multiple spatial scales, for different economic activities, and for a diversity of institutional regimes. The new synthesis is an empirical principle describing the probability that a region enters-or exits-an economic activity as a function of the number of related activities present in that location. In this paper we summarize some of the recent empirical evidence that has generalized the principle of relatedness to a fact describing the entry and exit of products, industries, occupations, and technologies, at the national, regional, and metropolitan scales. We conclude by describing some of the policy implications and future avenues of research implied by this robust empirical principle. C. A. Hidalgo and P.-A. Balland-Contributed equally.

Megacity growth in the developing world is fueled by a desire to access their large local labor markets. Growing megacities suffer from high levels of traffic congestion and pollution, which degrade local quality of life. Transportation technology that allows individuals to access the megacity without living within its boundaries offers potentially large social benefits, because individuals can enjoy the benefits of urban agglomeration while not paying megacity real estate rents and suffering from the city’s social costs. This paper presents evidence supporting the claim that China’s bullet trains are playing this role. The bullet train is regarded as one of the most significant technological breakthroughs in passenger transportation developed in the second half of the 20th century. Starting in 2007, China has introduced several new bullet trains that connect megacities such as Beijing, Shanghai, and Guangzhou with nearby cities. Through facilitating market integration, bullet trains will stimulate the development of second- and third-tier cities. By offering households and firms a larger menu of location alternatives, bullet trains help to protect the quality of life of the growing urban population. We document that this transport innovation is associated with rising real estate prices in the nearby secondary cities.

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The practice of split households among rural-urban migrants in China has persisted for more than twenty years. In this paper we compare three forms of split households, differentiated by whether the migrant's spouse and children are left behind or have joined the migrant: sole migration, couple migration, and family migration. Our survey of fifty chengzhongcun (urban villages) in Beijing conducted in 2008 shows that couple migration and family migration are outcomes of rural Chinese actively rearranging their household division of labor in order to maximize earnings from urban work opportunities. Migrants' decision to leave the children behind or to bring them along depends on the children's age and whether migrants' parents are available to help. Contrary to expectation, more household members in the city signals neither stronger intention to stay nor greater trust in the host society. These findings highlight the importance of thinking about migrants as circulators and thinking about migration, including family migration, as not necessarily a prelude to permanent settlement. They also underscore the need to address the experiences of long-term split households, a discourse that has received much less attention than one that assumes that a family lives together most of the time. Migration, settlement, and split householdsMigration is increasingly seen as not just a one-way move from the origin to the destination but as an activity that engages both origin and destination societies. Studies on international and transnational migration in particular have highlighted the multiple identities and ties that are stretched across nations and facilitated by

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