Mothers alter their speech in a stereotypical manner when addressing infants using high pitch, a wide pitch range, and distinct timbral features. Mothers reduce their vocal pitch after early childhood; however, it is not known whether mother's voice changes through adolescence as children become increasingly independent from their parents. Here we investigate the vocal acoustics of 50 mothers of older children (ages 7-16) to determine: (1) whether pitch changes associated with child-directed speech decrease with age; (2) whether other acoustical features associated with child-directed speech change with age; and, (3) the relative contribution of acoustical features in predicting child's age. Results reveal that mothers of older children used lower pitched voices than mothers of younger children, and mother's voice pitch height predicted their child's age. Crucially, these effects were present after controlling for mother's age, accounting for aging-related pitch reductions. Brightness, a timbral feature correlated with pitch height, also showed an inverse relation with child's age but did not improve prediction of child's age beyond that accounted for by pitch height. Other acoustic features did not predict child age. Findings suggest that mother's voice adapts to match their child's developmental progression into adolescence and this adaptation is independent of mother's age.
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