SINGAPORE - Thirty million words - that was the eye-popping figure quoted in the mid-1990s by American researchers Betty Hart and Todd Risley, for the word gap that develops by age four between children from low-income homes and their peers in higher socio-economic groups.
The two researchers, who spent 2½ years studying children, found that a child whose parents are highly educated and working professionals is exposed to roughly 1,540 more spoken words an hour than a typical child on welfare - with some caveats throw in.
Over time, they concluded, this word gap snowballs so much that by age four, children in rich families would have heard 30 million more words than children in poorer ones.
Moreover, it was not just the number of words, but also the type of words that varied. There was a substantial gap in the complexity of words being used.
There were also differences in the kinds of words and statements heard. By age three, children of professionals had heard 500,000 encouragements and 80,000 discouragements on average. In contrast, welfare children heard an average of 75,000 encouraging phrases and 200,000 discouraging ones.
Some of the families in the original study were recruited for a follow-up study when the children were in third grade. Researchers found that measures of accomplishment at age three were highly indicative of performance at age 10.
The professionals were giving their children an advantage with every word they spoke, and the advantage just kept building.
In recent years, though, their findings have been disputed, with subsequent studies showing that the "word gap" may not be as wide as the two researchers found.
But whatever the figure, most researchers studying early-childhood literacy still maintain that a significant language-development gap exists between different groups of children and that there is a correlation between poor early- literacy skills and lifelong academic challenges.
In 2008, for example, a Harvard University study found that types of conversations differ significantly between low- and high-income families - in part due to the differing levels of education attained by the parents in these groups.
In this study, the researchers reported that higher-income parents used longer sentences and wider vocabulary than the lower-income ones.
Another study led by Massachusetts Institute of Technology cognitive scientists in 2018 found that back-and-forth conversation between a child and an adult is more critical to language development than the word gap.
In the study of kids aged four to six, differences in the number of "conversational turns" accounted for a large portion of the differences in brain physiology and language skills. This finding applied to children regardless of parental income or education.
The findings suggest that parents can have considerable influence over their children's language and brain development by simply engaging them in conversation, the researchers say.
"The important thing is not just to talk to your child, but to talk with your child. It's not just about dumping language into your child's brain, but to actually carry on a conversation with him," said Dr Rachel Romeo, lead author of the paper.
Experts say there is a specific reason why hearing those words before age four is so important. Early childhood is a critical time in children's brain development. More than 85 per cent of a child's physical brain growth occurs in the first three years of life.
The great news is that parents, given the right knowledge and skills, can easily support their child's literacy and brain development.
One of the most successful programmes in the United States helping children with early-literacy development is The Thirty Million Words initiative. The programme started by Dr Dana Suskind, a paediatric surgeon at the University of Chicago, aims to teach parents and early childhood caregivers how to nurture brain development through frequent, quality communication.
It recommends parents use the "three Ts" when engaging with their young children - tune in, talk more and take turns.
Tune in by paying attention to what your child is communicating to you. This includes responding to babies' coos and cries with spoken language. Get down on your child's level. Maintain comfortable eye contact. Show your child you are interested in what he is saying.
Talk more with your child using descriptive words to build his vocabulary. Think of yourself like a sportscaster, narrating your child's day. These do not need to be exciting times, necessarily, but just adding words to the daily routines. "Okay, it's time to go grocery shopping. Let's find our shoes. Your shoes are pink. My shoes are black." And so on. Add "big" words to your speech. "You saw that big tree. It is humongous."
Take turns by encouraging your child to respond to your words and actions. Think of a conversation with your child like playing a game of catch. You want the ball to go back and forth. Support your child engaging in the conversation. Ask open-ended questions instead of questions that have a yes or no answer. Reflect back to your child what you hear him saying. "It sounds like recess was really fun today. Tell me more about the game you played."
As Dr Suskind told The Straits Times: "Children aren't born smart. They are made smart by their parents talking with them."
She adds: "Thirty Million Word Gap - it's like a headline for a news story. It grabs the attention of people. But behind it is this rich science that shows how the way parents interact with their children affects the way the children's brains develop.
"It is really about helping parents understand the important role they can play."