1. Data The data shows a clear pattern of rise and fall for the 'need NEG' construction. Its use was moderately common in the early 19th century, before rising to a peak frequency in the period from the 1850s to the 1880s. Following this peak, the construction entered a period of steady and significant decline throughout the entire 20th century. By the 2000s and 2010s, its usage had fallen to a small fraction of its historical high, becoming relatively rare in the language.

2. Possible explanation The variation is best explained by a long-term grammatical shift in the status of the verb 'need'. In the 'need NEG' construction (e.g., "You need not go"), 'need' functions as a modal auxiliary verb, similar to 'can' or 'must'. It is negated directly with 'not' and takes a bare infinitive. The data strongly suggests that this modal usage of 'need' has been undergoing a process of regularization. Over the past century, 'need' has been increasingly reanalyzed and treated as a standard lexical verb. As a lexical verb, it no longer behaves like a modal and requires different grammatical structures for negation.

3. Competing constructions The primary competing construction that has replaced modal 'need not' is the use of 'need' as a lexical verb, which requires 'do-support' for negation. This results in the now-dominant forms "do not need to" and its contraction "don't need to" (e.g., "You don't need to go"). This construction fulfills the exact same communicative function of expressing a lack of necessity or obligation. Additionally, other expressions like "don't have to" have likely absorbed some of the functional territory once occupied by "need not."