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 socioeconomic 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.