Singing in just intonation causes pitch drift when modulating through keys

Evidence

When singing a cappella, though, there is no requirement to stick to equal tempered tuning, and the possibility exists for singers to make use of the more consonant just intonation where the intervals are maximally consonant (see Section 3.3.3), for example with perfect fifths (frequency ratio 3:2) and just major and minor thirds (frequency ratios 5:4 and 6:5, respectively). Bearing in mind that just intonation cannot be used to tune a 12-note chromatic musical keyboard if the octaves are to remain in the ratio 2:1 (see Section 3.4), there could be musical situations in which singing in just intonation causes the overall pitch to drift as a piece modulates through different keys and back to the starting key. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1376

Categorization

• DEFINE: Explains just intonation via concrete frequency ratios, clarifying the term with examples.
• QUESTION: Why does pitch drift specifically accumulate when returning to the starting key—is drift direction predictable by interval choice?
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1376