Parasitic gap
In the study of syntax, a parasitic gap is a construction in which one "gap" appears to be dependent on another "gap", that is, the one gap can appear only by virtue of the appearance of the other gap, hence the former is said to be "parasitic" on the latter, e.g. Which explanation did you reject __1 without first really considering __2? While parasitic gaps are present in English and some related Germanic languages, e.g. Swedish, their appearance is much more restricted in other, closely related languages, e.g. German and the Romance languages. An aspect of parasitic gaps that makes them particularly mysterious is the fact they usually appear inside islands to extraction. Although the study of parasitic gaps began in the late 1970s, no consensus has yet been reached about the best analysis.
The following b-sentences illustrate typical parasitic gaps. The parasitic gaps are marked with a p-subscript:
The a-sentences are normal declarative sentences that contain no gaps at all. Each b-sentence, in contrast, contains two gaps, whereby the second gap is parasitic on the first. The c-sentences illustrate that if there is no "real" gap (that corresponds to the wh-expression in bold), the parasitic gap is not possible. One interesting thing about parasitic gaps like the ones here in the b-sentences is their motivation. Their appearance appears to be reliant on syntactic movement (e.g. wh-movement or topicalization). The fact, however, that there are two gaps in each b-sentence but only one fronted wh-expression is a source of the difficulty associated with the construction. How does it come to pass that one fronted wh-expression is capable of licensing two gaps? Another interesting fact about parasitic gaps is that they usually appear inside extraction islands (as they do in the examples just given), hence one might expect extraction from the site of parasitic gaps to be altogether impossible. The fact that the islands are ignored is a second source of challenge associated with the phenomenon.
The phenomenon of parasitic gaps appears to have been discovered by John Robert Ross in the 1960s, but remained undiscussed until papers by Knut Tarald Taraldsen and Elisabet Engdahl explored the properties of the phenomenon in detail. The knowledge of parasitic gaps was central to the development of the GPSG framework (Generalized Phrase Structure Grammar) in the mid 1980s, this knowledge then being refined later in the HPSG framework (Head-driven Phrase Structure Grammar) of Carl Pollard and Ivan Sag. In the 90s, a debate centered around the best theoretical analysis of parasitic gaps (extraction vs. percolation), this debate culminating in a collection of essays edited by Peter Culicover and Paul Postal in 2001.
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