Nationality as mass noun

A question on ELL recently aroused a discussion.

It's about usage of nationality as mass noun:

Like the Chinese, the Italian is a born gambler.

"You want to know why the Italian is skinny?"

The asker says, in a comment:

I was once told that we can think of other species collectively as thus we can use either The hippopotamus (singular) or The hippopotamuses (plural), but we can't do that with people.

I have never heard of this rule. Sure stereotyping is a slippery slope but not forbidden as such. What's with this rule? Am I missing something? Is this usage grammatically correct but politically incorrect or something like that?

Solution 1:

There are two different parts to this.

In the first place, the use of the for a typical member of some set (usually of people or animals) is grammatical and traditional, though rather literary and old-fashioned:

The lion is a carnivore.

The maple is a handsome tree.

The more usual construction now is to use the plural (without the):

Lions are carnivores.

Maples are handsome trees.

National adjectives to some degree follow this, though there are a number of complications, which I won't go into here. But in most cases, it is fine to use one as a singular noun:

A German came up to me in the street.

or as a plural, definite or indefinite:

The Spanish like to take their siesta.

What is not now socially acceptable is to use them in in the singular with the "typical" sense, eg The German to mean a (stereo-)typical German person.