Play a C major triad — C, E, G — and most listeners, trained or not, will call it bright, settled, maybe even cheerful. Drop that middle note by a single semitone, from E to E flat, and nothing else changes: same root, same fifth, same three notes stacked the same way. The chord now reads as C minor, and most of those same listeners will call it sad, or at least unsettled. One semitone is the entire difference between the two moods, and the note that moved is doing almost all of the emotional work.
The interval that's actually in charge
A triad is built from a root note plus two more, stacked in thirds. The interval between the root and that middle note is the whole story: four semitones up makes a major third, and three semitones up makes a minor third. Everything people describe as a triad's "quality" — bright versus dark, resolved versus unresolved — traces back to that one interval, not to the root note you started on or the fifth sitting on top, which stay identical in both chords. Swap only the third and you've swapped the chord's entire emotional register without touching two-thirds of its notes.
Harmonically, a plausible reason the major third reads as more "resolved" is its relationship to the overtone series: a vibrating string or column of air naturally produces a set of overtones above its fundamental pitch, and the interval that lines up most closely and simply with that natural pattern is the major third. A minor third sits at a slightly more complex, less naturally-occurring ratio relative to the same series. Consonance built on simple frequency ratios reading as more "settled" than consonance built on more complex ones is a real acoustic phenomenon — whether that acoustic fact is what actually produces the emotional label "happy" versus "sad" in a listener's head is the part that's much harder to prove.
Why "almost everyone" isn't "everyone"
Listeners raised entirely outside Western tonal music don't reliably sort major and minor the same way listeners raised on it do, and that gap is the strongest evidence that at least part of the major-happy, minor-sad association is learned convention rather than a fixed property of the sound itself — centuries of Western film scores, pop songwriting and hymnody pairing minor triads with grief and major triads with resolution, until the pairing feels innate to anyone who grew up inside that convention. The acoustic difference between the two thirds is real and measurable. The emotional label attached to that difference looks like a mix of something the ear responds to and something a culture trained the ear to expect.
What we're still unsure about
How much of the major/minor emotional split is "nature" — a response wired into how humans process pitch and consonance — versus "nurture" — a convention absorbed from a lifetime of exposure to music that pairs them that way — is a genuinely open question in music psychology, not one this post can resolve. Cross-cultural studies point in both directions depending on the population and the specific test used, and treating either side of that debate as settled would be overselling what the research actually shows.
This sits inside Chords, Triads & Seventh Chords, one of eight topics in Music Theory, one of four domains in Music, one of seventeen subjects the app can quiz you on.