Need to is less ambiguous than have to or got to about the source of the obligation; need to is said to require the obligation to stem from someone’s priorities or internal needs, whereas have to and got to can tie the obligation to any contextually plausible source. This paper investigates the social reasons that a speaker might choose or avoid the less ambiguous form. In view of the semantics of need to, the speaker who utters you need to unambiguously acts as if she is familiar with the hearer’s priorities and licensed to tell him what is good for him – a socially risky move. I therefore predict that you need to will be more appropriate and thus more common from people with knowledge about the relevant domain, people in authority over the hearer, and people who play a mentoring role in the hearer’s life because these people are more likely to be licensed to tell the hearer what is good for him in the context. I find evidence consistent with these predictions by investigating corpora that contain information about how the speaker and hearer relate to each other.
"Strong Necessity Modals: Four Socio-pragmatic Corpus Studies,"
University of Pennsylvania Working Papers in Linguistics:
2, Article 10.
Available at: http://repository.upenn.edu/pwpl/vol21/iss2/10