Where do these early differences come from?

A growing body of evidence suggests that a huge influence on early language development is the number of words that children hear as infants and toddlers. The more that parents speak to their infants and in front of their infants, the better infants get at understanding speech and learning words.

This issue has been explored in some previous work that has compared children who grow up in low socioeconomic status (SES) and high SES homes. An interesting paper in the November, 2013 issue of Psychological Science by Adriana Weisleder and Anne Fernald examined this question just within a sample of low SES Spanish speaking homes in the United States.

They had 19-month-old infants wear an audio recorder for at least one full day. Many infants wore the recorder for several days, and the longest recording day was selected.

Using software, the recordings were analyzed to identify all of the words spoken in the infants’ presence during that day. In addition, the researchers classified the speech by whether it was directed at the infants or whether it was just speech that the infants overheard.

Both when the infants were 19-months-old and again when they were 24-months-old, the researchers measured their efficiency at understanding speech. In these tests, the infants were seated in front of a screen. They saw pairs of pictures displaying common objects (like a dog or a ball). They heard the Spanish word for one of those pictures spoken and the researchers measured how much the infant looked at the picture corresponding to the spoken word as well as how quickly the infant looked at that picture after the word was spoken. In addition, at 24 months, the parents used a checklist to estimate the size of their child’s vocabulary.

Within this sample, there was a huge difference in the number of words that the infants heard. Some infants heard fewer than 2000 words in a day, while some heard over 15,000. In addition, there were big differences in child-directed speech. Some families spoke fewer than 1000 words to their children in a day, while others spoke over 10,000 words to their children.

The number of words spoken to children at 19 months was a significant predictor of the child’s vocabulary at 24 months. In addition, the number of words spoken to children predicted how quickly and effectively children looked at the picture associated with a word they heard. Statistical analysis demonstrated that the ease of identifying the words in speech was an important reason why infants who heard more words had a larger vocabulary at 24-months than infants who heard fewer words.

This early language experience compounds itself over time. Not only do infants who hear lots of words understand language better than those who hear fewer words, they are also more likely to start vocalizing and speaking words earlier. When children talk more, adults talk back to them more often. So, the early advantage in language ability gets bigger over time.

This research demonstrates the importance of a rich environment for infants. Infant brains are developing rapidly, and that brain development is strongly influenced by what is going on around them. The more that these infants are embedded in a complex language environment, the more that their language abilities develop. And that early development gives them a huge advantage as they start school.

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