The average toddler learns eight words every day in an explosive period of development that some researchers have described as a "word spurt."
Yet it's not just exposure to words, but the frequency of conversation with adults that predicts language skills and IQ a decade later, according to a new study published in the journal Pediatrics.
And a conversation does not necessarily have to involve words; even babies benefit when parents respond to their coos and grunts in a back-and-forth exchange that mimics conversation.
Jill Gilkerson, senior director of research and evaluation at the Boulder, Colorado, nonprofit that conducted the study, said the findings have implications not only for parents, but for child care providers, given that some young children spend more than half of their waking hours in day care.
"Child care is not about just feeding and diapering. This is a crucial period for development. And that has to be a focus for these kids and their brains," Gilkerson said.
In the study, which spanned 10 years, parents of newborns in the Denver area were given recorders about the size of a credit card, which were inserted into pockets on the children's clothing. The devices monitored conversation between the child and adults once a month for 12 hours a day, for a period of six months between birth to 36 months.
Software developed by Gilkerson's employer, the LENA Research Foundation, later analyzed the recording to determine the number and length of conversations in which both an adult and the child took turns communicating.
The researchers later examined results of language and cognitive tests given to 143 of the children when they were between 9 and 13 years old. They discovered that early results were predictive of outcomes 10 years later, most significantly for the period between 18 and 24 months.
The strength of the association at that age was surprising to Gilkerson and her co-investigators, who had expected to see a correlation between frequency of conversation between parent and child and later outcomes. Previous research has produced similar findings, and even adults absorb more information when engaging in a conversation than if they're just listening to a lecture, she said.
"The same way with babies. If they're involved in the conversation, they're more likely to be paying attention," Gilkerson said.
"On the other hand, to me, it's astonishing that you can predict anything with just a recording of an 18-month-old, automatically analyzed. That's kind of remarkable," she said.
"And I am surprised that this window of time seems to be more important. I don't know what's going on with that. But there may be this window of time where the brain is maybe primed for the positive effects of early interaction. More research is really needed to figure that out."
The study is the latest to show the impact of early exposure to language in children's development. From the age of 18 months to about age 6, children learn about eight new words a day, according to Lila Gleitman, professor emerita of psychology at the University of Pennsylvania, and editor of "The Acquisition of the Lexicon."
Previous research has shown that the amount of exposure to words between ages 18 and 36 months is predictive of a child's IQ at age 3.
And in a study published Sept. 5 in the Journal of Neuroscience, researchers at Harvard University and Massachusetts Institute of Technology showed that children who had more back-and-forth exchanges (called conversational turns) with adults had greater activation of the area of the brain that handles speech production and comprehension, and "stronger, more coherent white matter connectivity" in the brain.
The association was present "independent of socioeconomic status and the sheer volume of adult speech," the researchers said.
The lead author of that study, Rachel Romeo, a postdoctoral fellow in translational neurodevelopment at Boston Children's Hospital and MIT, said it's increasingly clear that the quality of interaction between young children and adults provides a crucial foundation for later academic success.
"This is exciting evidence to support early childhood intervention programs to capitalize on this sensitive time window," she said.
Kathy Hirsh-Pasek, director of the Infant Language Laboratory at Temple University in Philadelphia, said the new study is "yet another demonstration of the power of conversation."
"Language is the single best predictor of how well you're going to do in preschool, at the doorstep of language, and all the way through formal schooling," she said, adding that research has shown that strong language skills impact not just reading, but also a child's math ability and social skills.
"But just getting awash in language because you turned on the television isn't going to do it," she said. "It has to be back-and-forth — what I call the conversational duet, because you can't do it alone."
The LENA Foundation and its technology grew out of foundational research that found a stark disparity in the vocabulary of children by their family's economic status.
The work of Betty Hart and Todd Risley, made famous by the phrase "the 30,000 word gap," resulted in the book "Meaningful Differences in the Everyday Experience of Young American Children."
Their work has been challenged recently by other researchers whose work suggests the gap is not as vast as Hart and Risley believed, and that they failed to take into account low-income children's exposure to bystanders and caregivers.
In the new study, however, conversation between young children and adults was found to be significant despite the socioeconomic status of the household, Gilkerson said, and the implications for parents are clear: Don't just talk to your child, but talk with your child, no matter the child's age.
Before your child can talk, that means paying attention to what your child is paying attention to and responding quickly to the child's non-verbal cues.
"Follow your child's leads. Talk about what they're looking at, because that means they're more likely to absorb the language," Gilkerson said. "Repeat and expand. Expand on whatever the topic is."
Hirsh-Pasek at Temple University said she's been able to have a conversation with her 10-week-old granddaughter just by responding to the baby's vocalizations, and in doing so, she's teaching her granddaughter the fundamentals of conversation.
With older children, parents should be careful to ask open-ended questions that expand conversation instead of shutting it off, Hirsh-Pasek said. She recommends that parents and caregivers take at least two minutes a day to pay attention to what the child is looking at, and comment on it, thus beginning a conversation.
And keep in mind a specific numeral: 5.
No more than five seconds should pass between a response in a "turn-taking" conversation between an adult and child to count as a beneficial exchange, Gilkerson said.
And Hirsh-Pasek said a conversation between and adult and child should consist of at least five exchanges.
"Strive for five," she said. "If you can get five back-and-forth turns, you'll be doing a conversational duet."