Halliday & Matthiessen (1999: 557):
Secondly, systems have varying probability profiles, so that (in terms of information theory) they carry differential loads of information: the skewer the probabilities of the terms in a system, the greater the redundancy that it carries — hence the less we need to attend to its unmarked state. For example: it has been found that, in an English clause, positive is about ten times as frequent as negative (Halliday & James, 1993). What this means is that we build in to our sense of a figure the presumption that something is or something happens, rather than that something is not or does not happen; extra work has to be done if a process is being construed as negative. The same applies to future tense, as we saw: extra grammatical energy is required to assign a figure to the future.