Fig. 4

F0 and intensity variability across age groups and conditions. Panel (a) shows a significant decrease in F0 between 9 and 18 months. Panel (b) presents the significant increase in F0 and intensity variability from 9 to 18 months and across all age groups (3 to 18 months). Panel (c) displays the significant interaction between parent (father/mother) and age, with higher variability for mothers, particularly at 18 months. Data points represent individual participants, and error bars indicate standard error. Differences across conditions are discussed in relation to parental roles and child age.