Translation (just because I'm nice ):
When the subject and attribute are two nouns that differ in number, the normal thing is to establish agreement with the plural element: My childhood is memories of a Sevillan yard; All that is fallacies; The first cause for regression of the species is the alterations in its habitat. Nevertheless, in some cases it is also possible to establish agreement also in singular, especially when one of the nouns has a collective meaning, or when, being a morphological plural, it refers to a unitary concept: Those who developed the culture of La Venta were people of Maya speech/tongue; The salary is three thousand dollars a month; The migas ruleras is a dessert that is reserved for dinner.
Of course the English examples don't make as much sense as the Spanish ones. Personally, I don't see es posible establecer la concordancia en singular as a rule, but as a tendency, especially in the cases discussed above. I for one don't like the sentence La gente son estudiantes, why not say (Ellos/Ellas) son estudiantes?
Brazilian dude