Boys are just as likely as girls to say “It’s not fair”, although they might differ on just what “it” is.
I accept that boys were brought up to expect to be treated harshly.
I don’t agree that girls were taught more about reciprocity and fairness.
On the contrary, many (most?) girls were brought up to expect preferential treatment, regardless of whether it was fair.
Boys were taught (or quickly learned) that, if they misbehaved, they would suffer punishment.
Girls were often taught (or learned) that, if they misbehaved, they’d again just be asked nicely not to do it again.
They just lived up to their expectations, whether they thought they were being treated fairly or didn’t enter into it, they were powerless to resist.