Maternal Responsiveness and Infants’ Language Acquisition Learning the first 50 words is a milestone in early language acquisition, as it predicts the arrival of the naming explosion and the multiword sentence a few weeks later. Researchers found that the 9-month-old infants of highly responsive mothers (top 10 percent) reached this milestone as early as 15 months. The infants of nonresponsive mothers (bottom 10 percent) lagged significantly behind.

Source: Adapted from Tamis-LeMonda et al., 2001, p. 761.