As pointed out by Michaelyus in a comment, this is covered for two languages (English and French) in the 2003 paper On the Semantic Range of the Plural by Wayne P. Lawrence.
Briefly, English and French have different rules: English plural (despite prescriptive rules to the contrary) means "not 1", while French plural means "2 or more". However, Lawrence runs through many cases, showing that it's not quite as simple as even that.
In more detail:
- In English, singular is used for the number 1 only.
- There is a prescriptive rule that "plural means more than one". But even in publications that try to enforce this rule, you find many exceptions like "0.278 cubic metres" and "zero kilometres per hour".
- Zero is plural. Lawrence implies that this may be due to analyzing it as "no" rather than as a number, but doesn't actually argue for that.
- Fractions where the numerator is 1 usually take singular when read as fractions: "one half second" or "a third of a second". There's one half, so it's singular.
- Decimals follow the same rule for many speakers. But some speakers use singular for decimals less than 1 whose last digit is 1: "0.31 degree" but "0.3 degrees". Presumably they're analyzing it as [[0 point 3 1] degree], treating 1 as the "head", but Lawrence doesn't get into the explanation.
- Negative numbers other than -1 are plural: "-1.9 degrees".
- -1 is sometimes singular, sometimes plural. Singular seems more common, but Lawrence points out that people have spontaneously said things like "It feels like minus one degrees." The difference seems to be whether you analyze "-1 foot" as [minus [one foot]] or "-1 feet" as [[minus one] feet]. It's not mentioned in the paper, but plural seems much more common when read as "negative one" than when read as "minus one" (which fits—it's harder to get the analysis [negative [1 foot]]).
- Imaginary numbers work like negative numbers: "2i feet". Although Lawrence doesn't mention whether i itself is plural or singular, to me, "i foot" sounds just wrong; "1i foot" is not as bad, but "1i feet" is still more natural. He also doesn't mention complex numbers, but I think it's pretty clearly "√2+√2i feet", even though the magnitude is 1; presumably we just analyze it as "√2 plus √2i".
- Plural seems to be the semantic default: indefinite numbers are plural, as in "Schools Attended" on forms.
- In French, singular is used for numbers less than 2.
- Unlike the English prescriptive rule, this seems to be the native French understanding of number.
- Zero is singular.
- Decimals less than 2 are singular—even in cases like 2•10-7 newton.
- Notice "1,6 million homme", where "million" is singular (because there are fewer than 2 millions), and then "homme" agrees with that. But, although Lawrence doesn't mention it, it's usually "1,6 million d'hommes": less than 2 millions, but more than 2 men.
- Negative numbers are usually plural if -2 or lower. So it's the absolute value that follows the "2 or more" rule. Lawrence suggests that these are always analyzed as [minus []], which is why the sign doesn't matter. Lawrence doesn't say whether the same applies to imaginary numbers (or complex, etc.).
- Plural seems to be the semantic default, used for unknown/indeterminate number, as in "n années".
The paper only briefly mentions other languages, and other number systems like singular/dual/plural. At the end, Lawrence argues that it's unlikely that any languages use a rule that sets a boundary that's not on a whole number (e.g., by rounding rather than truncating). But he doesn't tell us whether, e.g., there are languages that extend the French model (so singular=[0, 2), dual=[2, 3), plural=[3, ∞)) and others that extend the English (singular=1, dual=2, plural=everything else). So there's plenty of room for further data gathering.
There are obviously many things here that could be psycholinguistically tested, but Lawrence doesn't discuss that, except for one point: The number systems of English and French predate the understanding of negative numbers (not to mention imaginary numbers) as numbers, which explains the pull of the [minus [one degree]] reading over [[minus one] degrees]. So why do some English speakers in some cases say [[minus one] degrees]? That "may reflect a more 'advanced' stage of the development of the speaker's understanding of negative numbers".
I think the case of French "2•10-7 newton" is even more interesting. That clearly requires a plural in English. Are the French really analyzing a number in scientific notation as a quantity at the grammatical level, even though for -1.5 they analyze the "minus" as outside the quantity? It should be easy to test processing time, distraction errors, etc. between English and French speakers here, but I don't know of any attempts to do so.
