First impressions influence subsequent behavior, especially when deciding how much effort to invest in an activity such as taking an online course. In computer programming courses, a context where social group stereotypes are salient, social cues early in the course can be used strategically to affirm members of historically underrepresented groups in their sense of belonging. We tested this idea in two randomized field experiments (N=53,922) by varying the social identity and status of the presenter of a welcome video and assessing online learners' persistence and achievement. Counter to our hypotheses, we found lower persistence among women in certain age groups if the welcome video was presented by a female instructor or by lower-status peers. Men remained unaffected. The results suggest that women are more responsive to social cues in online STEM courses, an environment where their social identity has been negatively stereotyped. Presenting a male and female instructor together was an effective strategy for retaining women in the course.