To me, the biggest confounding variable is the race/culture of the researchers. From the supplementary material:
"We recruited eleven male and two female research assistants to perform the drop-offs. All research assistants were recruited from two German speaking universities and
born between 1985 and 1993." Seems somewhat fortuitous that German/Nordic countries uniformly performed the best. In many countries, they would stick out like a sore thumb. To make this study more complete, they really should have someone who looks and speaks more native do this as well. Especially as race/culture seems to be so highly correlated with the result, it is only natural to see whether factors like distrust/xenophobia play a part. I mean, some random stranger (possibly using a language translator app!?) tells you to do something with a package they drop off and then leaves very quickly. How I react would certainly depend at least somewhat on my impression of the person and how that brief interaction went.
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