Bursty, irregular speech input to children predicts vocabulary size

Children learn language by listening to speech from caregivers around them. However, the type and quantity of speech input that children are exposed to changes throughout early childhood in ways that are poorly understood due to the small samples (few participants, limited hours of observation) typically available in developmental psychology. Here we used child-centered audio recorders to unobtrusively measure speech input in the home to 292 children, aged 2-7 years, over 555 distinct days (approximately 8,600 total hours of observation, or 29.62 hours/child). These large timescales allowed us to compare how different dimensions of child-directed speech input (quantity, burstiness) varied throughout early childhood. We then evaluated the relationship between each dimension of input and children's concurrent receptive vocabulary size. We found that the burstiness of speech input (spikes of words) was a stronger correlate with age than the quantity of speech input. Input burstiness was also a stronger predictor than input quantity for children's vocabulary size: children who heard spiky, more intense bouts of input had larger vocabularies. Overall, these results reaffirm the importance of speech input in the home for children's language development, and support exposure-consolidation models of early language development.