We examined how social antecedents impact students' use of explanations versus evidence to justify arguments using "statistical discourse analysis" on 2028 postings from 87 graduate students in five courses, each participating in four online debates. The results show that students overall were much more likely to justify arguments with explanations than with evidence. Explanations were more likely than evidence to be used in postings by women, when students posted responses to messages that conveyed greater social proximity (using he/she/they and using you instead of we) or directed attention (there), when making posts in early parts of a discussion thread and in the opening argument. Evidence was more likely to be used when responding to messages from men and when making posts towards the end of each discussion thread.