[GinaS]

Originally Posted by Supperman

Because the writer thinks that there are plural types of peanut allergies.

It was once thought that all types of peanut allergies were life long.
However, recent studies show some types of peanut allergies were cured when the children grew up , though other types of peanut allergies were life long.

That was a very good interpretation, but I think it's a different problem here, grammatically speaking (from an allergist's point of view there are different sorts of allergies to something like a peanut that contains a number of possible allergens).

I think allergies is plural in the first case because they're talking about many cases of people who are allergic to peanuts, rather than many kinds of allergies, although they may well mean that too. I'm not sure the difference matters there. In the second case they used singular because they were trying to talk about the allergy that an individual has.

The problem here is using the plural children, and then using a singular noun as the thing being possessed by the plural. It's akin to the error of using their to avoid the awkward but inclusive his or her instead of defaulting to his when gender is unknown, and probably why they incorrectly chose the singular allergy. It's a usage that's become so common, some grammarians are even reluctantly beginning to accept it under some circumstances.

But I think the sentence should have been written, "...children may outgrow their peanut allergies." Or, "...a child may outgrow his or her peanut allergy." That's the sort of sentence where more and more you will see even professional writers replace his or her with their as if it were a singular possessive, sacrificing grammatical correctness in exchange for political correctness and the flow of the sentence.