How does horizontal transmission complicate the reconstruction of a text?

In textual criticism, horizontal transmission—also known as an "open" or "contaminated" tradition—complicates the reconstruction of a text by undermining the fundamental assumptions of the genealogical or stemmatic method.1

The genealogical approach relies on "vertical" transmission, which assumes that a copyist followed only one model. Under this condition, critics can construct a stemma codicum (family tree) to trace witnesses back to a single archetype and discard redundant copies.1

Horizontal transmission occurs when a copyist uses multiple exemplars or copies a manuscript that has already incorporated variants from different sources. Because this process allows multiple textual states to coexist within a single witness, it becomes difficult, or even impossible, to map the relationships between manuscripts accurately. Consequently, the critic cannot rely on the simple elimination of variants based on a clear family tree and must instead resort to alternative strategies, such as controlled eclecticism or selecting a single representative witness as the basis for the text.1