Net outward migration is the main driver of negative population growth. A brain drain that began in the early 1990's has continued. Opportunities offered to Bulgaria's best and brightest by the 2007 European Union accession have only exacerbated the trend. Twenty percent of Bulgarian college students currently go abroad to study, and of those, approximately 80 percent remain abroad, according to sociologist Zhivko Georgiev of Gallup International. In recent years Bulgaria has also lost blue collar workers to EU countries such as Spain, Greece and Italy, where they can more than triple their salaries. Polls show a majority of these laborers intend to return to Bulgaria eventually, but it is doubtful whether these returnees would be sufficient to reverse current trends. To date, anecdotal evidence suggests only a small influx of returning workers to the country, despite the current global economic downturn.