Learn English – How does “each” change “are” to “is”

In the sentence They gave [object], the object is singular when the group collectively gives a single thing. In your example, They gave their word, the group collectively makes the same promise. In contrast, They gave their lives says that each member of the group devoted or sacrificed their individual lives – even if they shared a common cause.

This is true for all transitive verbs. For example, if a group of comrades make a conference call (or even a series of calls) to a single person, They called their friend. However, if they each make individual calls to separate people, They called their friends.

EDIT: Also note that we don't always pluralize abstract nouns in English. The more abstract the noun, the more likely we are to use it collectively. That's why plural consciousnesses sounds awkward even when talking about multiple minds: We usually think of minds concretely, consciousness in the abstract.

In many cases, you can use a noun either way. Choosing to pluralize or not helps to emphasize whether you mean it concretely or in the abstract. For example, All presidents swear an oath to uphold the Constitution of the United States. This emphasizes that all presidents make an oath of the same nature, even though they all do it individually. They all give their word.

In contrast, we usually write that they devote their lives to upholding the Constitution to emphasize the individual nature of their contributions. You could write that as singular life instead to emphasize the common, abstract nature of their devotion, but we usually don't.

The oldest senses documented in OED 1 are transitive, dating from the end of the 16th century (“To admit to a university degree”) and early 17th century (“To qualify (a person) for a degree”).

However, intransitive senses—“To take a university degree”, “To qualify as...”—have been around since the early 19th century. Dr Johnson accounted a century adequate to establish a poet’s reputation, so I think two centuries will suffice to establish a new use for a word.

Remarkably, OED 1’s earliest citations for the intransitive senses are from the poet Southey; and it was in Southey’s circle that another innovation first emerged into written English, damned by contemporary grammarians as an “awkward perversion”, a “solecism too palpable to receive any favor”—the passive progressive construction.