#1572. なぜ言語変化はS字曲線を描くと考えられるのか[lexical_diffusion][language_change][speed_of_change][schedule_of_language_change]

　語彙拡散に典型的なS字曲線については，最近では「#1569. 語彙拡散のS字曲線への批判 (2)」 ([2013-08-13-1]) で，過去にも「#855. lexical diffusion と critical mass」 ([2011-08-30-1]) や「#5. 豚インフルエンザの二次感染と語彙拡散の"take-off"」 ([2009-05-04-1]) を含む lexical_diffusion の諸記事で扱ってきた．自然界や人間社会における拡散や伝播の事例の多くが時間軸に沿ってS字曲線を描きながら進行することが経験的に知られている．噂の広がり，流行の伝播，新商品のトレンドなど，社会学ではお馴染みのパターンである．
　だが，言語変化においてこのパターンが生じるのは（事実だとすれば）なぜなのか．合理的な説明は可能なのだろうか．ロジスティック曲線のような数学的なモデルを参照することは，合理的な説明のためには参考になるかもしれないが，そもそも前提となるモデルがロジスティック曲線であるかどうかは，[2013-08-13-1]の (2) でも触れたように，自明ではない．
　Denison (58) は，変異形の選択にかかる圧力こそが，slow-quick-quick-slow のパターンを示すS字曲線の原動力であるとしている．

Now, speakers reproduce approximately what they hear, including variation, and even apparently including the rough proportions of variant usage they hear around them. However, if there is some slight advantage in the new form over the old, the proportions may adjust slightly in favour of the new. Thus the status quo is not reproduced with perfect fidelity. The speaker has (unconsciously) made a slightly different choice between variants --- albeit a statistical choice, reflected in frequencies of occurrence. And this effect of choice is greatest when the two variants are both there to choose from. In the very early stages of a change, so the argument runs, the new form is rare, so the pressures of choice are relatively weak and the rate of change is slow. In the late stages of a change, the old form is rare, so that the selective effect of having two forms to compare and choose between is again weak, and once again the rate of change is slow. Only in the middle period, when there are substantial numbers of each form in competition, does the rate of change speed up. Hence the S-curve.

　変化の過程の最初と最後は，古形と新形の2つの variants のうちいずれかが圧倒的に優勢であるために，どちらを選択するかの迷い（すなわち圧力）は低い．だが，変化の過程の中盤で多くの選択がフィフティ・フィフティに近くなると，毎回選択の圧力が働くために，選択の効果が顕在化しやすく，変化のスピードが増すことになる．
　Denison (60) はまた，他の論者の議論に拠りながら，次のようにも述べている．

Suppose that the small impetus towards change has to do with some structural disadvantage in the old form . . ., then after the change had taken place in a majority of contexts, reduction in numbers of the old form would perhaps reduce the pressure for change, allowing the rate of transfer to the new form to slow down again. Or words that are particularly salient, or maybe especially frequent or infrequent, or of a particular form, might resist the change for reasons which had not applied --- or at least did not apply so strongly --- to those words which had succumbed early on. Even if the impetus towards change is not structural but to do with social convention . . . there would still be the same slow-down towards the end.

　同じ選択の圧力の議論だが，それは構造的であれ社会的であれ同様に見られるのではないかと示唆している．変化の最初と最後に関わるのが言語的 salience であり，フィフティ・フィフティ付近に関わるのが non-salience であるという対比も鋭い指摘だと思う．ただし，Denison の議論は，言語変化のS字曲線を説明するのに上記のような理屈を持ち出すことはできるものの，実際にはそううまくは進まないという結論なので，単純な議論ではないことに注意する必要がある．

　・ Danison, David. "Log(ist)ic and Simplistic S-Curves." Motives for Language Change. Ed. Raymond Hickey. Cambridge: CUP, 2003. 54--70.
　・ Rogers, Everett M. Diffusion of Innovations. 5th ed. New York: Free Press, 1995.

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