Defaults effects can be created by social contexts. The observed choices of others can become social defaults, increasing their choice share. Social default effects are a novel form of social influence not due to normative or informational influence: participants were more likely to mimic observed choices when choosing in private than in public (experiment 1) and when stakes were low rather than high (experiment 2). Like other default effects, social default effects were greater for uncertain rather than certain choices (experiment 3) and were weaker when choices required justification (experiment 4). Social default effects appear to occur automatically as they become stronger when cognitive resources are constrained by time pressure or load, and they can be sufficiently strong to induce preference reversals (experiments 5 and 6).