Why is it "gangster" rather than "gangsters"?

1. The suspect, along with his two younger siblings, became the most notorious gangster in the district.
2. The suspect, along with his two younger siblings, became the most notorious gangsters in the district.

I know that grammatically, it should be gangster and not gangsters because phrases such as “along with”, “together with” and “as well as” introduce a parenthetical phrase and do not modify the subject, but still I find this usage confusing. To me, the three brothers became notorious gangsters not just one.

Could anyone help provide more insight?

Solution 1:

Switching the example to illustrate:

The suspect, just like his two younger siblings, became a notorious gangster.

Here, it is more obvious that the complement should be singular like the subject, and that the parenthesis, which gives additional information rather than changing the subject, shouldn't affect concord.

The suspect and his two younger siblings became notorious gangsters.

This time, we have an obvious plural subject.

The example you give falls somewhere between 'obvious singular subject with additional information inserted by a parenthetical' and 'obvious plural subject'. It has become standard to use grammar stressing the former here.

Solution 2:

Actually, neither form of this sentence sounds right to me.

As you mention, and as Edwin Ashworth points out in his answer, "The suspect, along with his two younger siblings, became the most notorious gangsters in the district" sounds wrong because the parenthetical should not affect the concord.

But "The suspect, along with his two younger siblings, became the most notorious gangster in the district" sounds wrong to me because "the most notorious gangster in the district" can only be a single person. To me, the use of "along with" implies that the younger siblings not only accompanied the suspect, but that they participated in the action described later in the sentence. But logically, it's impossible for them to participate in the action of "becoming the most notorious gangster in the district"... it sounds to me like you're saying they became the same person as their brother.

So I don't think this kind of sentence sounds good with either a plural or a singular noun. I only find it natural if the verb phrase following the parenthetical doesn't specify the number; something like: "The suspect, along with his two younger siblings, embarked on a life of crime."

Note: For some reason, "The suspect, just like his two younger siblings, became a notorious gangster" sounds completely fine to me. But "along with" and "just like" are two different phrases. I'm not sure why there's a difference for me, but I would guess it is because "along with" implies a single action carried out by a single group, while "just like" implies separate actions carried out individually by different people.