Their data, which took six years to analyse, showed that professional parents spoke almost 1,500 words more words every hour than unemployed parents. Thus a child in a professional family heard 11 million words a year, while a child growing up on benefit heard just three million.
Hart and Risley also analysed the type of language used and found that children from professional families received 700,000 words of encouragement, with just 80,000 negatives. The child from the family on benefit heard 60,000 words of encouragement and 120,000 negatives. The Hart-Risley research appears to confirm, again, the damage that deprivation can do to a child. Certainly for Mike Carden, of Netherley Valley Sure Start, it is a significant factor. A former docker who lost his job in the bitter Liverpool docks dispute of 1995, he has seen the decline of the area go hand in hand with the decline in children's language skills.
"Part of the reason is mass unemployment. People no longer work in factories, they no longer have apprenticeships where an older man or woman will shout at them and tell them how to behave properly. The only thing that is important is to be a celebrity, and most of the celebs have difficulty stringing a sentence together." Mr Carden reckons around 18,000 people live in the Netherley Valley, and about 10 will go to university.
"Education used to be something the dockers would aspire to. It isn't an aspiration any more."
For Janet Cooper, of Stoke Speaks Out, cultural factors play their part.
"As a society we expect so much of communication, but we don't place a great deal of value on it. For example, we expect to be ignored at the supermarket check-out, and we eat with the food on our laps in front of the television." Also, families are more isolated now so knowledge isn't passed down the generations the way it used to be. "There are a lot of families who don't look out for the milestones. If their child doesn't talk at two, well, none of the other kids in the street do either, so it must be normal."
Meaningful Differences in the Everyday Experience of Young American Children by Betty Hart and Todd Risley (Brookes Publishing)