I'm thinking perhaps the difference between Barkov and the Selänne kids might be that in the US, almost everyone is from "everywhere else". People tend to hold on to their original language and culture a bit more, I reckon. In Finland, the inhabitants are mostly Finnish, and perhaps there is more pressure to fit in. Especially since it's not very cool to be a Russian teenager at a Finnish junior high...
Don't know though, everyone is unique of course, and one's identity is a tricky thing. Hockey creates a shared experience for the players and spectators, allowing them to feel united, creating a national identity so deeply connected with sports. If Alexandr Jr grew up cheering for Leijonat with his friends rather than for Team Russia, which is likely, he might not have given one single thought to playing for the latter, no matter who his father is. Similarly, I have a hard time imagining the Selänne kids wearing USA jerseys at a USA-Finland match... In fact I pray that's not the case

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