The study included 29 children, 19-months-old, from low-income Hispanic families. Each child was fitted with a small audio recorder that captured all the sounds he or she heard during the day in their homes.
The recordings were analysed to distinguish between adult speech directed at the toddlers and speech they only overheard, such as when a parent or other caregiver was on the phone or talking with another adult.
The researchers found large differences between families in the amount of child-directed speech that toddlers heard from adults. One child heard more than 12 000 words of child-directed speech in a day, while another heard only 670, according to the study released online recently in the journal Psychological Science.
"That's just 67 words per hour, less speech than you'd hear in a 30-second commercial," study co-author Anne Fernald, a psychology professor at Stanford University, said in a news release from the Association for Psychological Science.
The researchers conducted language skills tests of the children five months later. At age 24 months, those who had experienced more child-directed speech had larger vocabularies than those who heard less child-directed speech.
It's known that children in poorer families generally have poorer language skills than those in wealthier families. These new findings help reveal why that's the case and suggest ways to reduce that language gap.
The American Academy of Paediatrics has more about language development.