Why does agreement between independent manuscripts strengthen evidence for a reading?

In textual criticism, the agreement between independent manuscripts serves as a critical tool for reconstructing an original text, primarily through the genealogical or stemmatic method.

When manuscripts are independent—meaning they do not share a common ancestor other than the original or a lost archetype—their agreement provides strong evidence for a reading because it is statistically and historically improbable that different scribes would independently introduce the exact same error or variation. By identifying shared variants, critics can construct a family tree (stemma codicum) of the manuscripts. In this framework, if multiple independent branches of the tree contain the same reading, it is inferred that the reading was present in the common ancestor (the archetype) from which those branches descend.1

This process allows critics to discard redundant witnesses that merely repeat the testimony of others, focusing instead on the agreement of independent lines to certify a reading. However, this method assumes "vertical" transmission, where a copyist follows a single exemplar. If a tradition is "contaminated" or "horizontal"—where copyists used multiple sources—the reliability of this agreement becomes more complex, requiring critics to exercise cautious empiricism and judgment of intrinsic merit.12