Oftentimes major keys are called "happy" and minor keys are "sad". Why is this? Is it universal across cultures that use these scales? Do similar connotations exist in other scale systems?
(If this isn't universal, then perhaps someone could explain which cultures it is sad in and why.)
The major key is present by nature in every note that is played. Therefore, it is interpreted as normal behavior, a happy day in our lives, 'cause that's what we expect to happen.
The minor key is opposed to the major key and it's perceived by us (without being aware) as if there was something wrong, hence sadness or restlessness.
Further explanation: When you hear a note, what you're really hearing are vibrations. For each note there is a whole set of vibrations that take in place, and that we don't even perceive.
When you play a low C, you're not hearing only C, but every other harmonic or overtone that belongs to C. That is, going from lower to higher in pitch, the low C being played, then C (octave), then G, then another C, then E, so on and so forth, each time being less the distance between the current overtone and the next one.
As you may have noticed, this first 5 notes (C, C, G, C, E) form the major triad. This means that by nature, the major triad is always present.
The note that results in the minor triad is E flat, which is the 18th overtone in the harmonic series. For this reason, and because the major triad is always there, it results in a contradiction that our human nature understands as sadness, unconformity, and restlessness.
If you want to go deeper into this, I strongly recommend Leonard Bernstein's lectures "The Unanswered Question", in which he engages in a deep and detailed explanation on this and other similar matters.
Here lies musical universality.
Edit: It can be considered universal, as physics have demonstrated its existence in nature and, as the same Bernstein says, the major triad (and even until the pentatonic scale, which would user overtones 5 and 6) can be found in every culture around the world.
They are not quite "universally" regarded as sad. For instance, in the Amazon jungle (particularly the part in Brazil) and some places in the Middle East, people use minor keys for happy songs of rejoicing. Basically, some experts argue it has to do with the qualities of natural speech. Here's an explanation:
http://philipball.blogspot.com/2010/01/is-minor-key-music-sad-for-everyone.html
I wouldn't say this is universal at all. For example, many songs in the Jewish musical tradition are written in minor modes yet convey joyful emotion. And what little I understand about Indian classical music is that it's scale-based, not key-center based.
This has been asked a lot on the net and it's complex to answer. It's partly cultural and partly psychological.
As Edgar Gonzales said in his answer, there's some explanation in the harmonics of the notes. What he said is correct, but he doesn't take into account temperament. Western music is based on Equal Temperament and as such, the perfect progressions of the harmonics of a note are not being respected, ruining perfect harmony.
Also, it's personal, there are many major songs that sound sad for me (in a way that disgusts me a little), and it also depends on the intetion of composer.
Personally, the major scale sound dull to me, and sometimes can be sad (in a dull way), but for me the minor scale is introspective and looking inside is something many people don't want to face (they may encounter sadness inside, I don't know).
If you want to dig into the rabbit hole, Indian Classical Music is based on the concept of Raga, a very complex one not found in Western music. It's a modal framework and as such, handles very complex "scales" and ways of travelling trough that arrangement of notes and singing melodic movements. Along with the concept of raga is the concept of Rasa, the effects and moods the raga creates on the listener, like love, heroism, introspection, wonder, and they create them on everyone universally, like musical archetypes. There are ragas that share the same Thaat (scale) but generate different rasas because of how they arrange the same notes in different melodic movements (based on a series of rules for each raga). Also can vary slightly from composition to composition. Take for example raga Asavari, Jaunpuri and raga Darbari Kanada:
Asavari:
Rasa: This raga evokes the moods depicting yearning for love, anguish, and melancholy. (source)
Jaunpuri:
"Jaunpuri (or Yavanapuri) is a morning raga, which describes a young, sensual and beautiful woman. An ancient Sanskrit text describes her as "... fully ripe, a foreign girl. Richly dressed, her hair plaited upon her brow, she wears golden ear-rings shaped like flowers and set with precious stones. Skillful, she plays in the morning languidly, sipping the wine of grapes, letting her white limbs and lovely form be seen." (source)
Darbari Kanada
Darbari Kanada is a raga to play in the night (it was played in the court of the Akbar emperor by the mythical singer Tansen, who created that raga) and is very introspective. Some relate it to spiritual devotion.
That's an example of the shades that the same scale may have based on different factors. As you know, art deals with our wordless, subconscious, symbolic area and it can have a thousand of colors.
(There's a lot of material that I'm slowly getting into it, so anybody let me know if I made a mistake.)
It is cultural. The ancient Greeks used a few more musical scales to denote certain moods (I'm missing my Greek history and theatre books at the moment). I'd say that minor keys sound sad because you've associated them with "sad", not that they cause humans to be sad.
I wouldn't categorize the minor scale as "sad", and I don't think it's universally sad, I would rather call it emotional or this kind of category. It's not universal because of a convention, it's just true fact that minor songs are more touching soul than major (in most cases).
The effect of the minor scale when listening to, makes you feel emotional, the human brain interprets this effect this way by nature. And this applies when played in a dance or fast songs even happy ones as well. Take Brahms Hungarian Dances for example, or zillions of Klezmer and Jewish dances that most of them are in minor, while being jolly.
So for conclusion, the right word isn't sad, but rather emotional/touching/moving.
Of course it would then be much easier to make you sad with touching scale which is minor than with major. And BTW there also many sad songs in major too.
It's not universal, it's my ears and your ears, my brain and your brain, it's nature.
Most answers focused on the harmonic aspects of this perception. What about the melodic ones?
Nobody knows the real answer. It's probably because of a mixture of unrelated psychological and cultural factors. But we can speculate and here's my speculation:
There is a -probably far from universal- tendency of equating rising sounds with happiness and falling ones with sadness. It may be related to the intonational patterns of languages (think of the English disappointment exclamation, a falling "aaah"). 80s video games took advantage of this by using rising sound effects ("wooeey") for positive things and falling ones ("yeeoow") for negative ones.
Now, raised scale degrees are said to have an ascending tendency while lowered ones are said to have a descending tendency. For example, the minor sixth in the natural minor scale has a tendency to resolve towards the perfect fifth whereas the major sixth of the melodic minor scale has a strong ascending tendency, either to a minor seventh (as in dorian) or a major seventh, which in turn would have an even stronger tendency to resolve to the tonic.
This tendency, in addition to the western music, is also recognized in classical Turkish music. I'm sure it's not nearly as universal as, say, octave equivalency, but it's nevertheless present in more than one musical culture.
We can define and classify heptatonic scales with the quality of intervals of the scale degrees in relation to its tonic. Scales dominated with major and augmented intervals tend to ascend, while scales with minor and diminished intervals tend to descend. Now, the natural minor scale has more descending tendencies with its minor 3rd, 6th and 7th than the major scale with its major 3rd, 6th and 7th.
I speculate that these two tendencies can be at the heart of this perceived "sadness" of minor.
Does this mean that scales with even more minor or diminished intervals would sound "sadder"? For example what about the phrygian that has an extra minor second compared to the minor? Does it sound sadder? It does indeed, at least to my ears (influenced by western and Turkish musical traditions). And yes, I find locrian even more depressing with its lowered fifth compared to phrygian.
Does this also mean that descending melodic lines are "sadder" than the descending ones, regardless of the scale? Once again, it does indeed, at least to my ears. Analyze those sad major songs and happy minor songs and, chances are, you'll find mostly descending melodies in the happier ones and mostly ascending melodies in the sadder ones.
The answer is still not known. There plausible explanations but we don't know why we associate certain feelings with certain sounds in a rigorous fashion. Same holds for memory allocation for certain smells etc. So every pseudo-scientific analysis relies on certain preassumptions on the cultural habits such as Minor=sad ...
It is generally agreed in Western music that minor is sad or serious or even melancholy, but is it the note order? I don't think so,as a thread of notes in a minor tune will contain the same sort of intervals as in a major tune. Maybe it's the implied or underlying chords that accompany the melody.Minor chords or harmony convey these feelings.Which is rather odd, as a major triad (CEG)consists of a major third and a minor third, whereas a minor triad (CEbG) contains a minor third followed by a major third, which just adds to the confusion - why should a min. chord sound any more 'minor 'than a maj. chord, seeing that both contain both intervals? So, would a diminished chord sound even more minory,(with all min.thirds) or an augmented more majory? The answer is - probably no-one knows.....
I would say it limits creativity in composition a tremendous amount to think of minor and major keys in such a stereotypical way. Minor keys to me sound minor and major keys major.
Whether a piece is sad or happy can be influence by the choice of minor / major but it is not defined by it. From my own instrument you can take for instance El testament d'Amèlia which is written in d minor. It is a piece written almost as a funeral song and is indeed very sad but the capricho arabe is also written in D minor (For the first part at least) but with the aid of some types of phrasing garners an effect that is while holding to the character of the minor keys still not sad.
Oftentimes major keys are called "happy" and minor keys are "sad". Why is this?
It's a conventional interpretation in Western classical music, or rather a simplistic interpretation of some conventions in Western classical music.
It's not hard to convince yourself of this - you can start by observing that the traditional minor scale in Western music is just a permutation of the traditional major, so the same relationships are present in both scales. Similarly, the major triad is composed of a major third and a minor third, while the minor triad is composed of a minor third and a major third, so again the same relationships are present.
You can also observe that in many styles of music, "minor" keys are used to compose joyful and upbeat music. There are a number of examples already cited, I'll add Irish traditional dance music, which has both melancholy airs in major keys and perky reels and jigs in minor keys.
Here is an explanation of WHY. It comes from the tensions among the notes based on the ratios of their frequencies.
These intervals in Equal Temperament are pretty close to whole-number ratios, so we can look at the tensions these ratios create to understand why the Major sounds generally more confident and established, while the Minor sounds perhaps more internally conflicted and less resolute.
A Major triad is near the ratios 4:5:6, in which the interval of the Perfect Fifth (2:3) between the root and fifth of the chord are the strongest interval (most consonant thanks to the lowest whole-number ratio), and the Major Third is the next most consonant. It suggest a group of three in which the root is in charge, the fifth is very supportive, and the third is in harmony with them both (also having a consonant 5:6 relationship with the fifth). Imagine a government where the VP backs up the Pres and so does the House Leader.
A Minor triad, however, is near the ratios 10:12:15 (since the Minor Third has a 5:6 ratio), meaning that after the Perfect Fifth the third and fifth share greater harmony (4:5) than the root and its third (5:6). Though the triad is still very harmonious, it doesn't all relate to the root, so there isn't the same confident consensus with both the fifth and third "going along with" the root. The root can be imagined as internally challenged by the fifth. Imagine a government where the VP backs up the Pres, but the House Leader shows more support to the VP, so there's a bit less consensus and strength.
