They're all European countries, which is not exactly a random selection of countries.
edit:
European union countries share similar populations both culturally and biologically.
If you moved the populations of Catalan, Cameroon, Kuwait, Cambodia, Korea and Kiribati to the same geographic area, that would be a different experiment.
"There is a significant variance for GDP (PPP) per capita within individual EU states, these range from €11,300 to €69,800 (about US$15,700 to US$97,000). The difference between the richest and poorest regions ranged, in 2009, from 27% of the EU27 average in the region of Severozapaden in Bulgaria, to 332% of the average in Inner London in the United Kingdom. On the high end, Inner London has €78,000 PPP per capita, Luxembourg €62,500, and Bruxelles-Cap €52,500, while the poorest regions, are Severozapaden with €6,400 PPP per capita, Nord-Est with €6,900 PPP per capita, Severen tsentralen with €6,900 and Yuzhen tsentralen with €7,200."
There is no way to isolate the effect of immigration on GDP from the effect of other policies of the EU, or the effect of the global economy, technological advancements, economic policy changes etc...
edit:
European union countries share similar populations both culturally and biologically.
If you moved the populations of Catalan, Cameroon, Kuwait, Cambodia, Korea and Kiribati to the same geographic area, that would be a different experiment.