I’ve been drawn to the “common cultural attributes” thesis just based on the observation that Nordic pop culture (Max Martin, Stieg Larsson, Ida Maria, Robyn) penetrates the Anglosphere very easily and has done so for a long time (Abba, Aha, Ibsen). It still strikes me that the most plausible mechanism here has to do with corruption and good government rather than individualism per se. I imagine that everyone looks out for his or her own interests, but the question becomes what does that balance with. If you balance it with fairly abstract principles of correct conduct, you get good government and enlightened self-interest. If you balance it with loyalty to extended family groups or long chains of personal connections, then you get corruption.
But that’s just ideas I made up.
Yglesias could read the very unfashionable Frederic W. Maitland. The Teutonic cultures all shared a weird set of inheritance structures, in which both collateral maternal kin and paternal kin could inherit. Lots of stuff follows from that.