Two main parts to language style are each implemented differently by the brain. For example, damage to Broca's area can lead to difficulty using function words while leaving use of content words unaffected; while damage to Wernicke's area can lead to difficulty using content words, while leaving function words unaffected.
Content Words label an object or action. They include Nouns, Verbs, Adjectives, and Adverbs.
Function words do not themselves carry much meaning, and are generally not noticed. However, they are used at a very high rate, and can tell us things about their user that content words can not.
These Twitter streams have been analysed to find the proportion of different word types. In the key, FPS = First Person Singular Pronouns ("me"); FPP = First Person Plural Pronouns ("we"); SP = Second Person Pronouns ("you"); TP=Third Person Pronouns ("they").
As an example, higher use of First Person Singular Pronouns has been associated with depression; lower use of First Person Singular Pronouns has been associated with self-confidence, and positions of power. Average use of "I" is 3.64%.