Sandra A. Thompson and Anthony Mulac,
"A Quantitative perspective on the grammaticization of epistemic parentheticals in English."
In Elizabeth Closs Traugott and Bernd Heine (eds.) Approaches to Grammmaticalization, vol. II: Focus on Types of Grammatical Markers, pp. 313-329.
The focus of this paper is the interplay between the phenomenon known in English as 'that-deletion', illustrated in (1) and (2), and the grammaticization of 'epistemic phrases' (EPs), illustrated in (2) and (3):
(1) I think that we're definitely moving towards being more technological.
(2) I think 0 exercise is really beneficial, to anybody.
(3) It's just your point of view you know what you like to do in your spare time I think.
Examples such as (1)-(3) have generally been thought of in terms of a process of 'that-deletion', i.e., an alternation between constructions like (1) and (2), with and without that. The purpose of this paper, however, is to suggest that what has been thought of as 'that-deletion' is better understood as an alternation between constructions like (1), in which I and think are main subject and verb, with that introducing a complement clause, and constructions like (2) and (3) in which I think is an epistemic phrase, expressing the degree of speaker commitment (Palmer, 1986: 51, Traugott, 1987), functioning roughly as an epistemic adverb such as maybe with respect to the clause it is associated with.