The pro cycle

The argument in this paper is based on the premise that null subjects are pronouns with no phonological substance, called pro, which may receive case and theta-role. Null subject languages may differ as to the referential properties of pro, in some languages pro may have specific reference, in others only generic or expletive reference, and again in others there is no pro at all. In some languages pro may also function as a complement. Pro is found in languages with or without rich verb agreement. There is thus no direct correlation between pro and subject-verb agreement. Many of the languages with obligatory subject (e.g. Scandinavian, English, French) have developed from earlier stages with null subjects. This can be described as a loss of pro from the lexicon of those languages. This loss can in turn be explained as caused by a lack of sufficient input data during acquisition; at a certain stage the necessary cues for a phonologically empty item is insufficient, and pro is lost from the language. This would also account for the apparent unidirectionality of the loss of null subjects. However, there are cases of null subjects as an innovation (e.g. contemporary colloquial French). Furthermore, since pro is such a widespread phenomenon cross-linguistically, there must be causes why it still exists after more than 100,000 years of language development, and why it originated in the first place. The paper will end by some attempts to explain such facts. Keywords: null subject; null object; empty categories; syntactic change; Scandinavian languages; Romance languages; Latin; Greek