Semantic logic of the word "both" in English - different from "and"?

Question

I am trying to figure out what additional semantic information "both" carries when used in a sentence. Does it differ from "and"?

Take the following sentences:

Alice and Bob both ate lunch.

vs.

Alice and Bob ate lunch.

What extra is gained by the "both" in the first sentence? Do either of the sentences give a stronger or weaker implication that the activity was performed together or separately? I've been overthinking it, so I don't know what my intuition tells me anymore. My initial thought was perhaps that using "both" added a stronger implication that the activity was shared between the two rather than separate events. If the word "together" is added to the end of either sentence, I feel that it feels a bit superfluous on the first, with "both" (yet not a questionable or starred sentence), and much more natural with the second, as in "Alice and Bob ate lunch together".

Looking at a dictionary entry, it would imply that "both" is used for emphasis, but gives the examples with "both" being in an initial position, which I feel automatically gives emphasis - "Both Alice and Bob ate lunch" seems to actually have a whole different feel to it, and to me would be used more to correct an incorrect assumption. "I thought only Bob was there..." "No, both Alice AND Bob ate lunch..."

Any thoughts?

Answer 1

Here’s what I gleaned from the relevant page in “Language Typology and Syntactic Description” (Timothy Shopen, ed.) (Volume II, Complex Constructions)

The semantic difference between normal coordination with “and” and emphatic coordination with “both…and” is that, in the latter, the separateness of the referents of the two coordinands is emphasized.

So, for example, “Both Wyoming and Wisconsin are in the USA,” emphasizes that, though both states are in the USA, logically neither or only one of the states might have been in the USA.

[I would add that emphasis on the separateness of Wyoming and Wisconsin might also be achieved by stressing "and": Wyoming AND Wisconsin are in the USA.]

The text I cite here points out that the sentence “Spanish and Portuguese are similar,” is felicitous, but the sentence “*Both Spanish and Portuguese are similar,” is infelicitous “because two things cannot be separately similar.” (The latter quote is on p. 55 of the book I mentioned above.)

These considerations apply to your examples. "Bob and Alice ate lunch," presupposes that both, one, or neither might have eaten lunch. But "Both Bob and Alice ate lunch" emphasizes the possibility that only one might have eaten lunch.