Lately, at our lab, we've been discussing the would-be content of our phd thesis: is it social science (social/cognitive psychology? sociology?), computer science... Well, it is abourt Human Computer Interaction definitely, that was where we reached in the discussion. HCI because it is really interdisciplinary. But what is a phd in HCI. I googled a bit and I found this paper: What is a PhD in HCI? by Chris Johnson (1996). Few quotes:

The criteria imposed in the field of Psychology cannot easily be applied to assess the quality of work in the field of Computer Science or Sociology. In consequence, external and internal examiners from individual disciplines cannot easily judge the quality of work that may draw upon several different fields.(...) Many Universities require that PhD candidates contribute to their field of research. (...) A second criteria for a doctorate in HCI is that the candidate shows a proper grounding in experimental techniques. (...) A third criteria that might be used to judge a 'good thesis' in HCI is whether the candidate contributes to the design and implementation of interactive systems. (...) An Understanding of Inter-disciplinary Research