The results might need a bit of unpicking, but there are some quite stark findings that the researchers, Jane Waldfogel and Elizabeth Washbrook, come up with:
Just under half (45%) of children from the poorest fifth of families were read to daily at age 3, compared with 8 in 10 (78%) of children from the richest fifth of families. Comparing children with the same family income, parental characteristics and home environments, those who were read to every day at age 3 had a vocabulary at age 5 nearly 2 months more advanced than those who were not read to every day.
Similarly, a child taken to the library on a monthly basis from ages 3 to 5 is two and a half months ahead of an equivalent child at age 5 who did not visit the library so frequently.
Regular bedtimes at 3 and 5 are associated with gains of two and a half months at age 5.
At the end of the summary of their findings, the researchers rate the factors that they have found as more or less responsible for contributing to the gap in test scores between children and decide that the most significant influence is in the category of "parenting and the home environment" (which includes reading to your child, the nature of the social interaction you have with your child etc.) while "material circumstances" (income, savings, pressure of bills etc.) is the second most significant factor.
The full report can be found here, a summary here and a BBC news story about it here. For anyone interested in other studies into the role of interaction and the development of children's language, have a look here at a 2009 blog post about gestures, here for a Polly Toynbee article about Hart and Risley's influential research into American children's language development, and here for a link to the Children of the Code site where Todd Risley is interviewed about his work