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There is, but it depends on the cruise as to how many conform. In Europe there will be a fair proportion take the trouble, but if your cruising out of the U.S. then NCL dress code of 'Optional Formal' is little more than a joke.
I have to disagree with Wilma, the few times I have been on NCL, I would say 75% at least were dressed up for formal night. Both were cruises out of the USA.
I agree with Aldrich, The American's enjoy dressing for dinner, but they do tend to change as soon as it's over.