The Learning Objective Governs Perceptual Narrowing: A Cross-Lingual, Layer-Wise, Ten-Seed Study of Self-Supervised Speech Encoders

Perceptual narrowing—the developmental loss of non-native phoneme discrimination in the first year of life \citep{werker1984}—is a canonical developmental finding, yet \emph{what learning objective produces it} remains open. We train a $\sim$7,M-parameter Transformer encoder on child-directed and read speech and evaluate phoneme ABX in English, French, and Mandarin over ten seeds, the seed as the unit of replication. Six results. $\mathbf{\text{(1)}}$~The objective sets the direction of cross-lingual transfer: reconstruction (masked mel-prediction) degrades non-native discrimination, prediction (frame-contrastive) improves it—a same-encoder, same-data gap of $+0.051$ in first-layer Mandarin ABX ($p=3\times {10}^{-8}$), unanimous in sign across twenty runs. $\mathbf{\text{(2)}}$~That decline combines a large arm-intrinsic difficulty gradient with a smaller language-specialization effect (matched vs.\ mismatched $+0.022$, $p={10}^{-4}$, all four layers). $\mathbf{\text{(3)}}$~Against a language-symmetric raw-mel floor, reconstruction pushes the first layer \emph{below} the discriminability of its input; prediction pushes it \emph{above}. $\mathbf{\text{(4)}}$~Read speech gives a $3.6\times$ steeper non-native decline than child-directed speech. $\mathbf{\text{(5)}}$~The customary three-seed budget cannot see this reliably: an effect unambiguous at ten seeds is called significant by as few as 70% of three-seed subsets. $\mathbf{\text{(6)}}$~Six objective configurations—sharpening, compression, consolidation, their composition, and word-level semantic grounding in two forms—fail to produce the full developmental signature (native improves \emph{and} non-native declines): a single objective moves both languages the same way because it acts on a shared representation. We conclude that the objective, not the architecture, is the first-order determinant of narrowing-shaped representational change.