Current research on ad copy design in the context of paid search highlights in particular the integration of an explicit call-to-action in the last part of the ad. Research on this topic is in its infancy and, so far, has only investigated click-through-rates rather than conversions. This paper considers conversion rates in ad copy design by examining the impact of a call-to-action on user behavior, conducted in a field experiment. Contrary to prevalent advice in the industry, our research shows that a call-to-action does not necessarily enhance paid search performance. Using logistic regression we identify a phrasing scheme which minimizes costs (clicks) while simultaneously maximizing profits (conversions) for adver-tisers. The diametric user behavior we observe provides first indicative evidence of a self-selection mechanism at play when paid search users respond to differently phrased ad copies. Future research in the field of paid search with regard to ad copy design should be cognizant of our findings.