Babies whose mothers speak to them often know 300 more words by age 2 than peers with less interaction.
Researchers found that when mothers frequently spoke to their infants, their children learned almost 300 more words by age 2 than did their peers whose mothers rarely spoke to them.
Huttenlocher et al., 1991. Early vocabulary growth: Relation to language input and gender. Developmental Psychology, 27, 236-248.