The flow of remittances, or money sent home by expats, is booming. In 2013 remittances dwarfed official aid by a factor of three. Last year totals grew 5%, to a record $435 billion; the World Bank says they could exceed $500 billion by 2017. Whether or not that happens, their impact will be huge. In 2012 the U.S. gave $209 million to Mexico in official aid but sent $22 billion in remittances—nearly 2% of the country’s GDP.
This story is from the March 15, 2015 issue of Fortune.