People sometimes make better decisions either by treating inanimate objects like strategic reasoners or by treating other reasoners as if they were inanimate objects. I advance ''strategic agenthood attribution'' as a psychological mechanism for modulating economic behavior. From a design perspective, inducing or suppressing the attribution of strategic abilities can influence aggregate social outcomes. Drawing from experiments in behavioral and institutional economics, developmental and social psychology, and human-robot interaction, I document human strategic reasoning in environments that mix typical adult humans with other kinds of entities. I then discuss anomalous misattributions of strategic agenthood and nonagenthood, with special attention to settings in which two types of error-either attributing strategic reasoning abilities to objects that can't think, or failing to attribute such abilities to typical human subjects-actually led to better economic outcomes. Because over-and under-mentalizing are endemic to human decision-making processes, game theory can be applied to improve interaction design, whether or not a given interaction is a game. I show that the flexible nature of human agenthood attribution increases the scope of game theory in some places, reduces it elsewhere, and generally introduces behavioral economics as a promising source of theory for human interactions with equivocal agents.