The persuasion literature has examined several mechanisms that have contributed to understanding the effectiveness of credible, attractive, similar, and powerful sources. These traditionally studied processes focus on how persuasive sources can affect attitudes by serving as peripheral cues or by influencing the direction or the amount of thoughts generated. After describing these processes that operate at the primary level of cognition, we review research on self-validation that demonstrates how and when source factors can affect a secondary cognition—thought confidence. Thought confidence refers to a metacognitive form of cognition. This recently discovered mechanism can account for some already established persuasion outcomes (e.g., more persuasion with high- than low-credibility or similar sources), but by a completely different process than postulated previously. Moreover, under some circumstances we have also been able to obtain findings opposite to those typically observed (e.g., more persuasion with low- than high-credible or similar sources). Our research reveals that a consideration of self-validation processes provides an integrative mechanism for understanding many other unexplored source variables, such as oneself as a source, source matching and mimicry, and threatening sources.