Despite many years of campaigning and action, adults struggling to read, write and count are still not receiving help - and are often not even aware they need it.
The latest study of nearly 900 Scots in their 30s reveals that 39 per cent of the men and 36 per cent of the women who took part had literacy abilities at "a level likely to impact on their employment opportunities and life chances".
In the case of numeracy, the problem was even more widespread: 65 per cent of men and 77 per cent of women experienced difficulties.
However, only a few of the adults surveyed were aware of their shortcomings: for reading, 4 per cent; for writing, 18 per cent; and for numeracy, 7 per cent. And, although a high proportion of these adults said they wanted to improve their literacy and numeracy skills, only a relatively small number had participated in relevant programmes.
"This suggests a need to explore the reasons for non take-up, to increase learning opportunities and to create innovative ways of engaging with potential learners," says the New Light on Adult Literacy and Numeracy in Scotland report.
Not surprisingly, the researchers found that adults with poor literacy and numeracy abilities tended to come from relatively disadvantaged backgrounds, where low expectations and low skills acquisition were prevalent across the generations.
"Family background, rather than school attended, appears to have a stronger impact on literacy and numeracy development," they say.
Adults with limited literacy and numeracy skills tended to have unstable working lives, to be living in overcrowded, rented housing and to feel disengaged from their local community. They were more likely to be receiving state benefits and the least likely to vote.
The research is part of a major study which began in 1970, tracking the lives of 16,500 people born in one week in April in Scotland, England and Wales. They are the BCS70 - the British Cohort Study of 1970 - group.
The latest part of this longitudinal study was carried out by the National Research and Development Centre for Adult Literacy and Numeracy in 2004 and focused on the literacy and numeracy skills of 891 adults in Scotland, who were part of the original survey and were aged 34 at the time.
They were assessed using multiple choice questions with levels of difficulty set against the Scottish Credit and Qualifications Framework levels 2, 3, 4 and 5 - in other words, from Access 2 to Standard grade Credit or Intermediate 2.
Literacy questions focused on reading comprehension but also covered writing composition, spelling, grammar and punctuation, and handwriting. Numeracy covered basic money skills, whole numbers, time, measures and proportion, weights and scales, length and scaling, and charts and data.
The gaps in achievement were evident from a very early age, the research shows. "We found that adults with the poorest literacy and numeracy had struggled at the very first stage of their formal education," the researchers say.
Those assessed as having SCQF level 2 or 3 (Access 2 or Access 3Standard grade Foundation) at 34 had gained the lowest scores in tests measuring their language development and visual-motor co-ordination at the age of five.
Those with skills at SCQF level 5 (Standard grade CreditIntermediate 2 or above) had achieved the highest test scores as five-year-olds.
The gap between the highest and lowest performers grew wider between the ages of five and 10.
"This indicates that the earlier difficulties are spotted and remedial action taken, the more likely it is that literacy and numeracy difficulties in adulthood can be prevented, or at least minimised," says the report.
It also suggests that teachers failed to spot some of the children's difficulties. Just 28 per cent of adults with the lowest literacy levels had received any remedial help with their reading, and only around 10 per cent of those with poor numeracy had received help with their maths by the time they were 10.
If these problems are not to be perpetuated, action needs to be taken to improve the literacy and numeracy skills of parents so that they can help their children learn, says the report.
"High-quality, community-based adult literacy and numeracy provision directed towards parents is therefore key to success," the researchers conclude.
A separate report, Estimating the Cost of Child Poverty in Scotland, also recommends targeting parents if children are effectively to be lifted out of poverty.
It found that the extra costs of providing services for children living in poverty range between pound;500,000 and pound;750,000 a year. In addition, the overall cost of the Neet group (not in education, employment or training) is roughly pound;1 billion a year, although not all those in this group are affected by poverty.
However, to reduce child poverty using income transfers would, initially, cost roughly pound;4,000 to pound;5,000 per child - the equivalent of pound;1 billion for all Scottish children in poverty. A more cost-effective measure would be to help parents get jobs and thus improve their children's long-term prospects, the report argues.
Early intervention might also lift children out of poverty, it adds, "but the financial benefits of this are more distant and much less certain".