Why is the first tone less common after m, n, r, l?

Question

In Gwoyeu Romatzyh (国语罗马字), the first tone uses the basic spelling except when the initial is a sonorant (响音) m, n, l, r.

For example,

ba (八) bar (跋) baa (把) bah (爸)

Sonorants mark the first tone by adding h an after the initial (聲母) and use the basic spelling for the second tone.

mha (妈) ma (麻) maa (马) mah (骂)

Tone 1 does indeed seem to be less common for m, n, l, r. For example, the syllable nü with the first tone (nǖ) does not appear to be a word.

Is this a real pattern? Is there an explanation for it based on how Standard Mandarin evolved from Middle Chinese (中古汉语)?

Answer 1

If you look up the tones for a random Chinese dialect, you'll probably see a chart like this that demonstrates the relationship between the dialect's tones and MC's tones:

This one is for Modern Standard Mandarin, and it provides a pretty clear answer to your question. All of the voiceless 平 syllables became 1st tone. All of the voiced 平 syllables became 2nd tone.

The only reason there are any 1st tone syllables with sonorant initials is sort of by accident. If you look up most of the examples on Wiktionary, it will give an expected Mandarin reflex that differs from the MSM pronunciation. (Also, several of them are just onomatopoeia.)

Answer 2

Middle Chinese Level Tone 平聲 split into 陰平 （voiceless initials) and 陽平 (voiced initials: M, N, L and R).

Middle Chinese 平聲 MNLR syllables almost all became 2nd tone in Mandarin. The minority of 1st tone MNLR syllables tend to be modern creations used in spoken Mandarin: 媽瞇摸；孬捏；撈;扔 Mā mī mō; nāo niē; lāo; rēng. There is thus a roughly 10:1 ratio of 2nd tone to 1st tone syllables.

Gwoyeu Romatzyh (promulgated in 1928) is a romanized companion for Bopomofo (1918), so it follows BPMF's practice of treating 1st tone as the default unmarked form, but there is one exception.

Because GR follows the Economy Principle (Keep, Change, Add), marking 2nd tone MNLR syllables would be wasteful:

(mh- = 1st tone) 媽瞇摸 mha, mhi, mho

(m- = 2nd tone) 麻蠻忙毛謀沒門盟埋迷民名棉苗模 ma man mang mao mou mei men meng mai mi min ming mian miau mo

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ECONOMY Principle

KEEP: Wherever possible, keep the basic unmodified spelling

CHANGE: Show tones by modifying a letter (especially a semivowel)

ADD: If modification is not possible, add a letter (2nd tone: Vr; 3rd tone: VV; 4th tone: Vh)