A 2017 survey of leading economists found that 34% of economists agreed with the statement "The influx of refugees into Germany beginning in the summer of 2015 will generate net economic benefits for German citizens over the succeeding decade", whereas 38% were uncertain and 6% disagreed. Studies of refugees' impact on native welfare are scant but the existing literature shows mixed results (negative, positive and no significant effects on native welfare). According to economist Michael Clemens, "when economists have studied past influxes of refugees and migrants they have found the labor market effects, while varied, are very limited, and can in fact be positive." A 2018 study in the "Economic Journal" found that Vietnamese refugees to the United States had a positive impact on American exports, as exports to Vietnam grew most in US states with larger Vietnamese populations. A 2018 study in the journal "Science Advances" found that asylum seekers entering Western Europe in the period 1985–2015 had a positive macroeconomic and fiscal impact. A 2019 study found that the mass influx of 1.3 million Syrian refugees to Jordan (total population: 6.6 million) did not have harm the labor market outcomes of native Jordanians. A 2020 study found that Syrian refugees to Turkey improved the productivity of Turkish firms.