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We now address the same set of questions about consistency, diversity, and social quality of voices that infants encountered in the music of their everyday lives. For the remainder of this “Voices Music Corpus” section, when we refer to “music”, we mean the cumulative duration of the music bouts that had exactly one unique voice.

How many voice types occurred in how many seconds of music?

To answer this first question, individual infants encountered from 1 to 66 different voice types (Median = 8, SD = 13.16; see Figure 3.7). Most infants encountered more than one specific voice in the music of their day, meaning that most infants encountered at least some diversity of voice types. One infant encountered exactly one specific voice in their musical input, reflecting maximal consistency of voice types. The cumulative duration of music ranged from 108 seconds to 5,616 seconds (Median = 1,077 seconds,

Figure 3.7. Almost all infants encountered more than one voice type in the music of their days.

Figure 3.8. The cumulative duration of music with exactly one voice ranged from 108 seconds to 5,616

seconds across infants. The red line indicates the median (1,077 seconds). This value was not correlated with the cumulative duration recorded or the cumulative duration of coded seconds.

Did infants encounter balanced frequency distributions of voice types?

To answer this question, we calculated several measures to index consistency and diversity of voice types that infants encountered in their everyday music. First, we constructed individual frequency distributions of voice types for each infant (see Figure

3.9). Based on a visual inspection, none of these individual frequency distributions of voice types appeared to be balanced such that each voice type occurred for an equal duration. Chi-square goodness-of-fit tests comparing each infant’s real frequency distribution of voice types to a balanced null distribution with the same number of voice types confirmed that none of infants’ real frequency distributions were consistent with a balanced null model (i.e., p < .05 for each infant’s test).

Figure 3.9. From a visual inspection, no infant encountered a balanced distribution of voice types. Each

plot depicts the frequency distribution of voices for an individual infant. The unique voice types are on the x-axis ordered by their rank-frequency. The height of each bar represents the proportion of the cumulative duration of all voice instances that was accounted for by each specific voice type. The gray horizontal line shows the balanced-null proportion. The individual plots are ordered by the difference between the rank- one proportion and the balanced-null proportion (largest to smallest, top-left to bottom-right).

To what extent was one specific voice more available relative to a balanced null?

As another index of consistency and diversity of voice types, we computed the proportion of the cumulative duration of music that was accounted for by the single top

voice type (rank-one proportion). For individual infants, the rank-one proportion of voices ranged from .08 to 1.0 (Median = .45, SD = .22). Thus, for most infants, one specific voice accounted for almost one-half of the cumulative duration of all voices – this signals consistency of voice types in individual infants’ everyday musical input. For each individual infant, we next calculated the proportion of one voice type if each voice had occurred for an equal duration (balanced-null proportion, Median = .13, SD = .20), and we compared this value to that of the infant’s real frequency distribution of voice types. The difference – the rank-one-consistency-bias for voices – ranged from 0 to .71 across infants (Median = .25, SD = .14; see Figure 3.10A). A paired t-test confirms that the actual proportion of the rank-one voice was significantly greater than the rank-one balanced-null proportion (t(33) = 12.06, p < .001).

Figure 3.10. Compared to a balanced null, infants’ single top-most voice (A) and the other voices that

occurred in the top of their frequency distributions of voice types (B) were much more available. The red lines show the median values.

voices (magenta points), the balanced-null proportion, and the cumulative number of unique voices that occurred in each infant’s day (see Figure 3.11). The distance from each point to the balanced-null line represents that infant’s rank-one consistency bias for voices that occurred in the music of their days. We observed the same pattern as we did for tunes, that all three measures decrease as the number of unique voices increases.

Figure 3.11. The main measures of consistency were each sensitive to the number of unique voices. The

rank-one proportion for voices (magenta points) and the balanced-null proportion are shown with respect to the cumulative number of voice types that occurred in the music of each infant’s day. The distance from each point to the balanced-null line is that infant’s rank-one consistency bias for voices.

To what extent were some voice types more available than other voice types?

Next, we examined the other voice types, in addition to the single top voice, that accounted for a greater proportion of the cumulative duration of voice music than would be expected if each unique voice occurred for an equal duration. Several infants (.29) encountered only one unique voice for a longer cumulative duration than would be expected by the balanced null, and therefore had no additional voice types in the “other- top-ranked” category. These infants were excluded from this analysis. For the remaining

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infants, the cumulative duration of these “other-top-ranked” voice types accounted for over one-third of the cumulative duration of music (Median = .35, SD = .13, range: .12 - .67; other-top-ranked proportion). One-fifth of a balanced null distribution would be accounted for by the same number of voice types as the number of “top-ranked” voice types (Median = .20, SD = .08; range: .09 - .38; balanced-type-proportion). The other- top-ranked consistency bias – the difference between these two values – was .16 (Median, SD = .08; range: .03 - .31; see Figure 3.10B). A paired t-test confirms that the actual proportion of the other top-ranked voice types was significantly greater than the balanced-null proportion (t(24) = 9.74, p < .001). This confirmed that many infants encountered several specific voices that were more available than other specific voices – a sign of consistency in the voices that infants encountered in their everyday music.