[qwerty1234599]

Scandinavian countries are not really countries. More like a family club. (This is starting to change though)

When everybody is just like you, it leads to high trust, no us vs them mentality. Same is true in eg. South Korea and Japan.

Contrast this with their similarily northern neighbour Russia, which is basically the America of eastern europe. A nation resulting from a melting pot of ethnicitiess, languages, cultures (most current Russians are really assimilated from smaller native cultures, their great grandparents didn't identify as Russian). And the end result? Corruption. Everybody just tries to milk public funds as much as they can, and so forth.

[trap_chateau]

While I'm not disagreeing with your proposed cause to high trust within a culture, I disagree with 'high diversity causing corruption'. I think the corruption issues in eastern europe are a whole different conversation and I'm not sure how diversity in ethnicity would necessarily cause that.

[burntoutfire]

Poland is very uniform in term of cultural background and origins (and everyone is white), and yet has very low trust levels.

[oblio]

Poland (and most of Eastern Europe) has been steamrolled repeatedly by neighboring empires which have imposed what amounted to foreign governments. These governments ruled despite, not due to, the local populations. Ergo secret police, network of informants, etc, just to control the population.

It's hard to have very high trust levels when your own government and state are working against you and when your coworkers and neighbors can at any time rat you out for unpatriotic activities.