Steven Vertovec explores the vast, often hidden networks of modern, global diasporas

Irish or Italian migrants to the US 150 years ago might have hoped that a letter would reach home weeks after it was posted in New York. Today's global populations can telephone their relatives or send an email.

These changes do not just keep them in touch with their families, they also enable migrants collectively to plan activities and distribute resources easily across the political and presumed cultural boundaries of nation-states. Ethnic groups, political movements, terrorist organisations and business corporations defining themselves by common causes, loyalties and interests can connect in real time around the world. As they do so, they develop fascinating long-distance ties that have a profound impact on politics, economics and society in their original and adopted countries. Such "transnational communities" not only spring from contemporary globalisation, they also act as catalysts for further globalisation. In the process, they foster new social structures, develop innovative business strategies, spread diverse cultural activities and forge different group identities. These enhanced global networks pose considerable political challenges to the countries where they operate, including the UK.

Until recently, we knew such networks were developing but had little research on their impact. For that reason, the Economic and Social Research Council set up the ?3.8 million national research programme on transnational communities in 1997. A conference in London today brings together the findings of 19 different projects, selected from more than 180 project proposals that draw upon many disciplines, including economics, politics, business studies, sociology, geography and anthropology. The studies tell us about how global social networks function and offer lessons on the impact on political lobbying, entrepreneurship, human-resource management, gender relations and notions of belonging and citizenship.

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The projects were broadly organised into four themes: migration, economics, politics, and society and culture. The first group demonstrated how a better understanding of the motivations and means of migration can inform and help shape policy. Annie Phizacklea and Bridget Anderson from Warwick University's department of sociology, for example, found that domestic work in private households is now the largest employment sector for mostly undocumented migrant women workers entering the European Union. Most remit money exclusively to female relatives rather than to their husbands or fathers. And while the UK government recently regularised many such workers, the initial effect was often to reduce their income as their employers resented the change of status.

Migrants almost always depend on social networks linking areas of emigration and immigration. Even refugees maintain considerable links with their places of origin. Richard Black (Sussex University), Khalid Koser (University College London) and Nadje Al-Ali (Essex University) showed the extent of political lobbying, economic investment and promotion of human rights among Eritrean and Bosnian exile communities in the UK, the Netherlands, Germany and the US. The ability of such refugees to participate transnationally in post-conflict reconstruction is directly related to policy decisions at home and abroad.

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The research programme was not just about migrants to western countries, it also looked at the nature of the "global employee" and how transnational corporations manage their staff and activities abroad, studying the economic relationship between local contacts and global networks. Jon Beaverstock, a geographer at Loughborough University, found that the City of London still exports more than 2,500 workers a year to the overseas outposts of major financial companies, while importing about 800 overseas workers. His studies in London, New York and Singapore suggest that the success of overseas postings relates directly to the degree to which expatriates embed themselves in local business networks. Beaverstock also shows how expatriates bring significant social, economic and cultural capital to their host environments but require better advice, training and mentoring before they start a typical three-year stint overseas.

Global networks pose many challenges for governments, such as the definition of citizenship where people have multiple allegiances. With the "war on terrorism", governments are increasingly tempted to try to constrain the cross-border activities of social groups and non-state organisations. So it is important to understand the contemporary political links between diasporas with homeland states. Eva ?stergaard-Nielsen, of the London School of Economics, compared the opportunities that the Turkish and Kurdish diaspora communities in Europe have to participate in politics in adopted countries such as Germany with their continued participation in politics "at home". "Migrant or refugee transnational political networks are not synonymous with terrorism or radical movement," she says. "Homeland politics is not just a matter of Molotov cocktails or demonstrations but also the politics of school curricula and mother-tongue teaching. Migrant organisations can also play an important role as bridgeheads between their members and the host country's political system."

Other projects looked at transnationlism and its effect on transnationals' sense of identity. One project, for example, examined changing modes of Islamic transnationalism. Through fieldwork in the UK, Lebanon and Dagestan, alongside detailed analyses of texts and websites, J?rgen Nielsen and his colleagues at Birmingham University developed an understanding of how an age-old Sufi form of social organisation can adjust to rapidly changing contemporary environments, relate modern electronic communications to more traditional Islamic media and function locally and transnationally.

Many of these findings have relevance to debates on issues from asylum to globalisation and will help to create a wider social perspective on a phenomenon that has until now been viewed more for its effect on finance than its impact on people.

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Steven Vertovec (University of Oxford) is director of the ESRC research programme on transnational communities. For more information: . The conference "People without Frontiers" takes place at Church House, Westminster, London SW1 on October 25 from 10.30am.