# Why do languages seem to lose the dual number in particular?

Proto-Indo-European is reconstructed as having a dual number; Ancient Greek and Sanskrit both had one, yet modern Greek and all Indo-Aryan languages have lost it; similar patterns can be observed in Semitic languages for example. What is it that causes to dual to frequently disappear from languages?

• Well, it is the rarest number, after all – you talk about one things or three-to-infinity things more often than you talk about two things specifically. Jul 2, 2023 at 20:07
• It's still present in Hebrew. Also partially in English, since "both" and "all" are very infelicitous when used of the wrong number of entities. Jul 3, 2023 at 13:37

I agree with the other answer but want to add two things. First, this is part of Greenberg's universal 34:

No language has a trial number unless it has a dual. No language has a dual unless it has a plural.

Second: in a formal analysis this fact can be accounted for using a feature geometry. Harley & Ritter (2002 in Language) propose the following for Person (the left-hand side), Gender ("class") and Number (the other parts of the "individuation" tree):

In this system, singular is represented as [+Minimal], and plural as [+Group]. The dual can be represented as [+Minimal,+Group]: it represents the smallest possible group. [+Augmented] is used for trials / paucals.

A feature geometry like this comes with some predictions. If a language marks some node N, it will also mark all parents of N. In acquisition, parents of a node N will be acquired before N itself. So the hierarchy predicts that the trial/paucal ([+Augmented]) cannot be acquired without first acquiring the singular.

Another prediction is that when a combination of independent features can be marked, each subset of this combination can be marked as well. For example, if [+Group,+Minimal] can be marked, it should also be possible to mark just [+Group] or just [+Minimal]. In this way the hierarchy predicts that no language attests a dual ([+Group,+Minimal]) without also attesting a singular ([+Minimal]) and plural ([+Group]). Furthermore, the plural will be acquired no later than the dual.

Now suppose a language has singular, plural, and dual. It is possible for children to acquire the language up to the point of the plural, without acquiring the dual. But it is not possible for children to acquire the dual without the plural or the singular. As a result, if any number is going to disappear, it's going to be the dual.

It's simply less salient than the others. If you're going to reduce your stock of numbers from three to two, you'll end up talking about one thing or many things more often than you talk about two things specifically. So it's the easiest one to lose.