A forerunner of the theory was proposed by the Danish linguist Holger Pedersen in 1951, but did not involve glottalized sounds. While early linguists such as André Martinet and Morris Swadesh had seen the potential of substituting glottalic sounds for the supposed plain voiced stops of Proto-Indo-European, the proposal remained speculative until fully fleshed-out theories were simultaneously but independently published in 1973 by Paul Hopper of the United States in the journal Glossa and by Tamaz V. Gamkrelidze and Vyacheslav V. Ivanov of the Soviet Union in the journal Phonetica in 1972.
The Glottalic Theory "enjoyed a not insignificant following for a time, and still has adherents; but it has been rejected by most Indo-Europeanists." The most recent publication supporting the Glottalic Theory is Bomhard (2008 and 2011) in a discussion of the controversial Nostratic hypothesis. An earlier supporter, Theo Vennemann, has abandoned the theory because of incompatibilities between it and his theory of the Semitic origins of Germanic and Celtic (e.g. Vennemann 2006).
The traditional reconstruction of Proto-Indo-European includes the following stop consonants:
|labials||dentals||palatalized velars||velars||labialized velars|
|breathy voiced stops||*bʱ||*dʱ||*ǵʱ||*ɡʱ||*ɡʷʱ|
*b is parenthesized because it is at best very rare and perhaps nonexistent.
Historically, this inventory was not introduced as an independent proposal, but instead arose as a modification of an earlier, typologically more plausible theory. In the original Proto-Indo-European proposal, there was a fourth phonation series, voiceless aspirated *pʰ, *tʰ, *ḱʰ, *kʰ, *kʷʰ, assumed to exist on the basis of what is found in Sanskrit, which at the time was thought to be the most conservative Indo-European language. However, it was later realized that this series was unnecessary and was generally the result of a sequence of a tenuis stop such as /t/ and one of the Proto-Indo-European laryngeal consonants[vague], in other words either *h₁, *h₂, or *h₃. The aspirate series was removed, but the breathy voiced consonants remained.
There are several problems with the traditional reconstruction. The first is the rarity of *b. From a typological point of view, if a single voiced stop is missing from a phoneme inventory (a 'gap'), it would normally be /ɡ/ that is missing; on the other hand, if a voiceless stop is missing, the labial /p/ is the most likely candidate. With ejectives, it is close to universal for a gap to be /pʼ/.
Secondly, there are few languages which have breathy voiced consonants but no voiceless aspirates, and yet fewer that simultaneously contrast breathy voice with full voice. Roman Jakobson has asserted that no such language is known; however, this is disputed by some linguists who oppose the theory. For example, Robert Blust showed that a system of voiceless, voiced and murmured stops, as postulated in the traditional reconstruction of Proto-Indo-European, exists in Kelabit, a language of the Sarawak highlands in Borneo. Others have observed, however, that the actual phonetics involved in Kelabit is not murmured but, rather, voiceless with breathy release, and, hence, this is not comparable to what is posited for Proto-Indo-European. In any event, the traditional reconstruction remains a typological oddity.
The third issue is a longstanding but unexplained observation of Indo-Europeanists about the distribution of stops in word roots. It had long been noted that certain combinations of consonants were not represented in Proto-Indo-European words. In terms of the traditional system, these were:
- No root contained a sequence of two plain voiced stops, that is, in schematic terms, there were no roots of the type **deg.
- No root contained both a voiceless stop and a voiced aspirate, that is, roots of the type **dʰek or **tegʰ were not attested.
- On the other hand, the plain voiced stops were compatible with either of the other two series: *degʰ or *dek were both possible.
These constraints on the phonological structure of the root cannot be explained in terms of a theory of assimilation or dissimilation, since they display a radical difference in patterning between two sets of consonants — the voiced stops — that ought to behave identically. Typologically, this is also very odd.
Original glottalic proposal
The Glottalic Theory proposes different phonetic values for the stop inventory of Proto-Indo-European:
|voiceless stops||p ~ pʰ||t ~ tʰ||k ~ kʰ||kʷ ~ kʷʰ|
|ejective or glottalized stops||(pʼ)||tʼ||kʼ||kʷʼ|
|voiced stops||b ~ bʱ||d ~ dʱ||ɡ ~ ɡʱ||ɡʷ ~ ɡʷʱ|
In his version of the Glottalic Theory, Hopper (1973) also proposed that the aspiration that had been assumed for the voiced stops *bʰ *dʰ *gʰ could be accounted for by a low-level phonetic feature known to phoneticians as "breathy voice".[clarification needed] This proposal made it possible both to establish a system in which there was only one voiced stop and at the same time to explain developments in later Indo-European dialects (Greek, Latin, and Sanskrit) that pointed to some kind of aspiration in the voiced series.
Gamkrelidze and Ivanov (1973, 1995:5-70) have posited that both non-ejective series (traditional *p *t *k and *bʰ *dʰ *gʰ) were fundamentally aspirated (that is, *pʰ *tʰ *kʰ and *bʰ *dʰ *gʰ, respectively) but had non-aspirated allophones (that is, *[p] *[t] *[k] and *[b] *[d] *[g]). According to them, the non-aspirated forms occurred in roots where two non-ejectives were present because of a rule that prohibited more than one aspirate in the same root. To express the variability of aspiration, Gamkrelidze and Ivanov write it with a superscripted h, for example *dʰ. Thus, an Indo-European *DʰeDʰ (where *Dʰ represents any non-ejective stop) might be realized as *DeDʰ (attested in Indic and Greek) or as *DʰeD (attested in Latin). In contrast, traditional theory would trace a form attested as both *DeDʰ and *DʰeD to an Indo-European *DʰeDʰ. The advantage of this interpretation over the previous is circumventing the typological oddity of the language having only voiced aspirates by identifying the voiceless non-aspirates of the traditional stop system (*p *t *k) as voiceless aspirates (*pʰ *tʰ *kʰ).
The phonation system proposed by the Glottalic Theory is common among the world's languages. Moreover, the revised system explains a number of phonological peculiarities in the reconstructed system. The absence of a labial plain voiced stop *b in the proto-language now becomes an absence of a labial ejective *pʼ, a rather proportionally more common state of affairs. The theory also provides a completely coherent explanation to the patterning of the stop series in roots (Hopper 1973):
- In very many languages that have glottalic consonants, there is a constraint against two such consonants in the same root. This constraint has been found in many languages of Africa, the Americas, and the Caucasus.
- If the "plain voiced stops" were not voiced, then the "voiced aspirated stops" were the only voiced stops. The second constraint can accordingly be reformulated as: Two nonglottalic stops must agree in voicing.
- Since the glottalic stops were outside the voiced/voiceless opposition, they were immune from the constraint on voicing agreement in (2).
Decem and Taihun
In 1981, Hopper proposed to divide all Indo-European languages into Decem and Taihun groups, according to the pronunciation of the numeral '10', by analogy with the Centum-Satem isogloss, which is based on the pronunciation of the numeral '100'. The Armenian, Germanic, Anatolian, and Tocharian subfamilies belong to the Taihun group because the numeral '10' begins from the voiceless t there. All other Indo-European languages belong to the Decem group because the numeral 10 begins from the voiced d in them. The question then can be framed as which, if either, of these groups reflects the original state of things, and which is an innovation.
Direct and indirect evidence
While the Glottalic Theory was originally motivated by typological argument, several proponents, in particular Frederik Kortlandt have argued for traces of glottalization being found in a number of attested IE languages, or the assumption of glottalization explaining previously known phenomena. This lends the theory empirical support. (Similarly, the laryngeal theory was proposed before direct evidence in Anatolian was discovered.)
Among the Indo-Iranian languages, Sindhi reflects the IE non-aspirated voiced series unconditionally as implosives. Kortlandt also points out the distribution of voiced aspirates within Indo-Iranian: they are lacking from the Iranian languages and the Nuristani languages, two of the three accepted main branches of Indo-Aryan, and within the third, Indo-Aryan, also lacking from Kashmiri, which he suggests points to voiced aspirates being an innovation rather than a retention.
In Germanic, Danish stød in certain dialects (vestjysk stød) corresponds with the Proto-Germanic voiceless stops, deriving from the allegedly glottalized PIE series. Kortlandt also proposes word-final glottalization in English to be a retention, and derives features such as preaspiration in the Scandinavian languages and certain instances of gemination in High German from preglottalization as well.
In both Latin (Lachmann's law) and Balto-Slavic (Winter's law), vowels are lengthened before a "voiced" consonant. This had always been somewhat puzzling. It is the same behavior that vowels exhibit before Proto-Indo-European laryngeals, which are assumed to have included a glottal stop. It may be that the glottalic consonants were preglottalized, or that they were ejectives that became preglottalized in Italic and Balto-Slavic before losing their glottalization and becoming voiced. It is very common in the world's languages for glottal stops to drop and lengthen preceding vowels. In Quileute, for example, the sequences VCʼV, VʔCʼV, and VːCʼV, as found for example in ak’a ~ a’k’a ~ āk’a, are allophones in free variation.
Dialects of Armenian also show glottalization. This has been argued to be influence from the other Caucasian languages, but Kortlandt argues glottalization cannot be considered a modern innovation and must be reconstructed with a wider dialectal distribution in older stages of Armenian.
The primary objection to the Glottalic Theory is the alleged difficulty in explaining how the sound systems of the attested dialects were derived from a parent language in the above form. If the parent language had a typologically unusual system, like the traditional p b bʰ, then it might be expected to collapse into more typical systems, possibly with different solutions in the various daughter languages, which is what one finds. For example, Indo-Aryan added an unvoiced aspirate series (/pʰ/), gaining an element of symmetry; Greek and Italic devoiced the murmured series to a more common aspirate series (*bʰ to /pʰ/); Iranian, Celtic, and Balto-Slavic deaspirated the murmured series to modal voice (*bʰ to /b/); and Germanic and Armenian chain-shifted all three series (*p *b *bʰ > /f p b/). In each case, the attested system represents a change that could be expected from the proposed parent. Now, if the system were typologically common, as proposed by the glottalic theory, then it might be expected to be stable and, therefore, to have been preserved in at least some of the daughter languages, which is not the case: no daughter language preserves ejective sounds where the glottalic theory postulates them. Glottalic proponents respond that, if Proto-Indo-European did not have true ejectives but rather some less stable kind of glottalic consonant, their loss would be more understandable; but this undercuts many of the original motivations of the glottalic theory, which are based on ejectives (rather than glottalized consonants) and on the idea of a typologically natural (hence stable) system. Regardless, there are languages where ejective consonants have voiced allophones, such as Blin and Kw'adza, and this has been suggested as an "empirical precedent" for the Glottalic Theory.
The typological underpinnings of the Glottalic Theory itself have also been questioned, for instance recently by Barrack.
Additionally, if traces of glottalic stops can be found in separate branches such as Italic and Indo-Iranian, the change of *p’ *t’ *k’ to *b *d *g must have occurred independently in each IE branch after their separation from the Proto-Indo-European matrix. Taking these as identical but independent innovations would, according to traditional models of sound change, be an astonishing coincidence, which most linguists would find very hard to believe. However, it cannot be assumed that Proto-Indo-European was a uniform language, and presumably a putative shift from ejective to voiced stops was already present as variation at an early stage. Kortlandt also asserts that the change from aspirated to plain voiced stops, which is likewise required as an independent change in numerous Indo-European branches under the traditional model, is not attested elsewhere and is typologically suspect. (However, this exact change has been observed to have taken place independently numerous times in the Indic languages.)
A compromise viewpoint would be to see the original formulation of Glottalic Theory (with ejective stops) as representing an earlier stage in the history of Proto-Indo-European, which would have undergone a period of internal evolution into a stage featuring unstable voiced glottalized stops, before branching out into the daughter languages. This would explain the root restrictions in Proto-Indo-European, the near-universal loss of glottalic consonants in the daughter languages, and the lack of *b in the traditional system.
A scenario of glottalic framework in pre-Proto-Indo-European, although possible, is at present unprovable by the methods of historical linguistics due to the uncertainty concerning the possibility of other languages or language families being related to Proto-Indo-European, which might be used as corroborating evidence; and in practical terms, it is irrelevant for the traditional reconstruction of Proto-Indo-European that describes only its latest stage (the so-called "Late Proto-Indo-European"). However, Kortlandt suggests that voiced aspirate was probably not in Indo-European before the division into the branches.
Fallon  has reviewed and discussed the arguments for and against the ejective model of Proto-Indo-European consonantism and has concluded that most of the objections raised against the Glottalic Theory are specious.
One objection that has been raised against the glottalic reconstruction is that the voiced stops are voiceless in some daughter languages: "unvoiced" in Tocharian and Anatolian, aspirates, later fricatives in Greek and Italic. Thus, some more recent versions of the Glottalic Theory hypothesis do not have voiced consonants at all, or treat voicing as non-distinctive. Such an inventory is:
|ejective or glottalized stops||(pʼ)||tʼ||kʼ||qʼ||kʷʼ|
(Here the traditional palatalized vs. plain velar dichotomy is treated as a velar-uvular contrast, as posited by Hopper 1981. This is not required for the Glottalic Theory, and it may have been allophonic at an early stage in the proto-language.)
- Holger Pedersen, Die gemeinindoeuropäischen und vor indoeuropäischen Verschlußlaute (1951), Copenhagen: Munksgaard.
- Benjamin W. Fortson IV, Indo-European Language and Culture (2nd edition, 2010), Malden, MA: Wiley-Blackwell Publishing, pp. 59-60.
- Robert Blust, 1974. A double counter-universal in Kelabit. Papers in Linguistics :309-24.
- This table reflects the views of Gamkrelidze and Ivanov
- Paul J. Hopper, 'Decem' and 'Taihun' Languages: An Indo-European Isogloss, in Bono Homini Donum: Essays in Historical Linguistics in Memory of J. Alexander Kerns, edited by Yoël L. Arbeitman and Allan R. Bomhard, Amsterdam: John Benjamins Publishing Company (1981), Part 1, pp. 133-142.
- Frederik Kortlandt, 'Glottalic Consonants in Sindhi and Proto-Indo-European', reprinted in Studies in Germanic, Indo-European, and Indo-Uralic, Amsterdam: Rodopi (2010), pp. 121--124. Counterarguments exist for his argument of no discernible substrate influence on the development of the specific Sindhi glottalization.[clarification needed] See also Michael Witzel, "Substrate Languages in Old Indo-Aryan", Electronic Journal of Vedic Studies 1999 (http://www.ejvs.laurasianacademy.com/ejvs0501/0501ART.PDF).
- Frederik Kortlandt, "Proto-Indo-European glottalic stops: The evidence revisited"
- Frederik Kortlandt, "How old is the English glottal stop?"
- Frederik Kortlandt, "The rise and fall of glottalization in Baltic and Slavic"
- Paul D. Fallon, 2004. "The Best is Not Good Enough". In Akinlabi & Adesola, eds, Proceedings: 4th World Congress of African Linguistics
- Charles M. Barrack, The Glottalic Theory revisited: a negative appraisal, Indogermanische Forschungen, 2002, pp. 76-95, and Charles M. Barrack, The Glottalic Theory revisited: a negative appraisal. Part II: The typological fallacy underlying the Glottalic Theory, Indogermanische Forschungen, 2003, pp. 1-16.
- Colin P. Masica 1991, The Indo-Aryan Languages, pp. 102-103.
- Allan R. Bomhard (2008 and 2011), for one, has tried to show that Proto-Indo-European was not, in fact, genetically isolated but, rather, that it was related to several other languages/language families of Eurasia, North Africa and the Middle East, the Indian subcontinent, and Northwestern North America.
- Paul D. Fallon,The Synchronic and Diachronic Phonology of Ejectives (2003), New York, NY, & London: Routledge, Chapter 6: Ejective Voicing.
- Robert S. P. Beekes, Comparative Indo-European Linguistics, 2nd edition revised and edited by Michiel de Vaan (2011), Amsterdam: John Benjamins Publishing Co., pp. 119 & 128-129.
- Charles M. Barrack, The Glottalic Theory revisited: a negative appraisal, Indogermanische Forschungen, 2002, pp. 76–95.
- Charles M. Barrack, The Glottalic Theory revisited: a negative appraisal. Part II: The typological fallacy underlying the Glottalic Theory, Indogermanische Forschungen, 2003, pp. 1–16.
- Robert S. P. Beekes, Comparative Indo-European Linguistics. 2nd edition revised and corrected by Michiel de Vaan, Amsterdam: John Benjamins, 2011.
- Allan R. Bomhard, Reconstructing Proto-Nostratic: Comparative Phonology, Morphology, and Vocabulary. 2 vols. Leiden: E. J. Brill, 2008.
- Allan R. Bomhard, The Nostratic Hypothesis in 2011: Trends and Issues. Washington, DC: Institute for the Study of Man, 2011.
- James Clackson, Indo-European Linguistics: An Introduction (Cambridge Textbooks in Linguistics). Cambridge: Cambridge University Press, 2007, pp. 40–49.
- Anthony Fox, Linguistic Reconstruction. Oxford University Press, 1995.
- Thomas V. Gamkrelidze and Vjačeslav V. Ivanov, Indo-European and the Indo-Europeans, translated by Johanna Nichols, 2 volumes. Berlin and New York: Mouton de Gruyter, 1995.
- Paul J. Hopper, Glottalized and murmured occlusives in Indo-European. Glossa 7:2:1973, 141-166.
- Theo Vennemann, Grimm’s Law and loanwords. Transactions of the Philological Society 104:2:2006, 129-166.