Given that the study was funded by the Swedish research council and by a Swedish professor from Lund, and that the participants were recruited randomly from people in a park (assuming one in Sweden) I would guess they were Swedish. I share your disappointment about the current cliquish political environment in US, but that may not be a good explanation of the results. It might well be that such behavior is universal, but that will need more substantiation/exploration.
I think the GP was just giving a local example (for them, at least) to support an argument for a universal phenomenon. They probably didn't mean to imply that anyone associated with the study was American or particularly influenced by American thought (or, for that matter, that tribalism itself is a uniquely American quality).
Right. That's exactly what I was saying. I think this is a universal phenomenon, for which we have clear evidence in front of our eyes played out on the most public of stages. It was not meant to link the US and the study, nor was it to indict/laud the US system. Merely a neutral commentary that popped into my mind reading this study. You said it more concisely than me - thanks :-)