They select for characteristically WEIRD people and exacerbate those tendencies further. The consequences are more significant than might be immediately apparent.
Interestingly enough, if you read Henrich, he says that the original form of WEIRDness was imposed by the Catholic Church on the Germanic peoples 100-1500 years ago, forbidding cousin marriage and other close marriages, forcing people to deal with, and trust, people outside their own clan. Whatever the problems with later forms of WEIRDness, this was a good thing.
Interestingly enough, if you read Henrich, he says that the original form of WEIRDness was imposed by the Catholic Church on the Germanic peoples 100-1500 years ago, forbidding cousin marriage and other close marriages, forcing people to deal with, and trust, people outside their own clan. Whatever the problems with later forms of WEIRDness, this was a good thing.