Much of the syntax literature notes an amnestying effect of a third wh-word (the so-called “additional wh-effect” [Bolinger, 1978; Kayne, 1983]) in questions that violate the Superiority Condition [Chomsky, 1973], as in (1) [Kayne, 1983; Clifton, Faneslow & Frazier, 2006].
(1a.) * What did who buy there? (1b.) ? What did who buy where?
However, psycholinguistics studies have not supported this observation [Clifton, Faneslow & Frazier, 2006; Fedorenko & Gibson, 2010]. We ask if an additional wh-phrase can ever improve acceptability of multiple-wh-questions, and if so, when. We believe such a study will help to reveal how the constraints that operate during language processing influence the way grammatical violations are comprehended and judged.
(2) The maitre d' tried to figure out what who ordered. (3) The maitre d' tried to figure out what who served to whom.
Fedorenko & Gibson  observe that the acceptability of (2), which has a monotransitive verb and two wh-phrases, is indistinguishable from (3), which has a ditransitive verb and three wh-phrases. However,  suggests, through an informal acceptability judgment, that the additional wh-effect is apparent in (4), in which only ditransitive verbs are compared.
(4a.) ? You know perfectly well where who put what. (4b.) * You know perfectly well where who put it.
My research on this topic reveals that the transitivity of the verb in question (e.g., ditransitive ‘put’ or monotransitive ‘buy’) and the lexical status of the additional item (e.g., a wh-word like ‘where’ or a pro-form like ‘there’) influence the change in acceptability between conditions.
- Ackerman, L. & M. Yoshida. (2016). Three wh-words are better than two (when violating the Superiority Condition). The 29th Annual Conference On Human Sentence Processing (CUNY16) – University of Florida, FL (March 2016). [poster, abstract]
- Ackerman, L. & M. Yoshida. (2015). A third wh-phrase increases acceptability of ditransitive multiple-wh-questions. The 89th Annual Meeting of the Linguistic Society of America (LSA2015) – Portland, OR (January 2015). [poster]
- Ackerman, L. & M. Yoshida. (2014). Adding a Third Wh-phrase Sometimes Increases the Acceptability of Multiple-wh-questions. The 20th Architectures and Mechanisms for Language Processing (AMLaP) – Edinburgh (September 2014). [poster]