  Generative AI systems have been heralded as tools for augmenting human
creativity and inspiring divergent thinking, though with little empirical
evidence for these claims. This paper explores the effects of exposure to
AI-generated images on measures of design fixation and divergent thinking in a
visual ideation task. Through a between-participants experiment (N=60), we
found that support from an AI image generator during ideation leads to higher
fixation on an initial example. Participants who used AI produced fewer ideas,
with less variety and lower originality compared to a baseline. Our qualitative
analysis suggests that the effectiveness of co-ideation with AI rests on
participants' chosen approach to prompt creation and on the strategies used by
participants to generate ideas in response to the AI's suggestions. We discuss
opportunities for designing generative AI systems for ideation support and
incorporating these AI tools into ideation workflows.
