Three studies tested the idea that our analyses of human behavior are guided by an "intentionality bias," an implicit bias where all actions are judged to be intentional by default. In Study 1 participants read a series of sentences describing actions that can be done either on purpose or by accident (e.g., "He set the house on fire") and had to decide which interpretation best characterized the action. To tap people's initial interpretation, half the participants made their judgments under speeded conditions; this group judged significantly more sentences to be intentional. Study 2 found that when asked for spontaneous descriptions of the ambiguous actions used in Study 1 (and thus not explicitly reminded of the accidental interpretation), participants provided significantly more intentional interpretations, even with prototypically accidental actions (e.g., "She broke the vase"). Study 3 examined whether more processing is involved in deciding that something is unintentional (and thus overriding an initial intentional interpretation) than in deciding that something is unpleasant (where there is presumably no initial "pleasant" interpretation). Participants were asked to judge a series of 12 sentences on one of two dimensions: intentional/unintentional (experimental group) or pleasant/unpleasant (control group). People in the experimental group remembered more unintentional sentences than people in the control group. Findings across the three studies suggest that adults have an implicit bias to infer intention in all behavior. This research has important implications both in terms of theory (e.g., dual-process model for intentional reasoning), and practice (e.g., treating aggression, legal judgments). (Contains 4 tables.)