It is easy to see that social robots will need the ability to detect and evaluate deceptive speech; otherwise they will be vulnerable to manipulation by malevolent humans. More surprisingly, we argue that effective social robots must also be able to produce deceptive speech. Many forms of technically deceptive speech perform a positive pro-social function, and the social integration of artificial agents will be possible only if they participate in this market of constructive deceit. We demonstrate that a crucial condition for detecting and producing deceptive speech is possession of a theory of mind. Furthermore, strategic reasoning about deception requires identifying a type of goal distinguished by its priority over the norms of conversation, which we call an ulterior motive. We argue that this goal is the appropriate target for ethical evaluation, not the veridicality of speech per se. Consequently, deception-capable robots are compatible with the most prominent programs to ensure that robots behave ethically.