Much of the time it comes down to things like church attendence. Yet all that really measures is church attendence, not religiosity per se. Americans tend to place greater emphasis on church attendence than other people, like those in Latin America - who usually attend church mostly for special occasions. Then in Europe we have the case of church attendence being down, but the number of pilgrimages to holy places(like Fatima, Lourdes, Vatican, etc) is at a high number.
A few years ago I read one study showing that Americans are more religious than European because they're more likely to read the Bible literally than Europeans. But taking the Bible more literal is not a sign of being more religious. I'm Catholic, and we tend to interpret the Bible more allegorically. And America has a stronger religious tradition of literalism than Europe.
So who really is more religious than the other? You really can't say. You have to take into account so many different contexts, it begs the question as to how the hell can you make a standardized definition of religiosity in order to accurately measure it to begin with.
And this is just ONE flaw I commonly find in these kinds of studies.
Yes this must be among your favorite hobbies: throwing around random arguments that have no valid conclusion.Yes, correlation does not always imply causation, which is why I had considered the nature vs. nurture argument but came to no valid conclusion.