MASTERING their mother tongue takes some English children an extra four years of learning because of the complexities of the language, say researchers.
The Institute for Learning at Hull University estimates that around one quarter of the school population needs the extra years to learn to write common words that have irregular spellings.
Children in other European countries with regular writing systems are not handicapped in this way, according to the university's Ken Spencer.
Reforming the spelling system could reduce literacy failure, he says. The alternative is to accept that, even with the best teaching methods, some rote learning of the most irregularly spelt but commonly used words is sensible.
The Hull team have developed a model for predicting spelling success, based on the length of words, their regularity and how frequently they are used. For example, "man" is an easy word but "through" - the same number of sounds but twice as many letters - is one of the hardest.
They tested the model against national spelling data for primary children, and were able to predict quite accurately how many pupils could spell a word from its make-up.
Further trials of primary children of different ages showed they found te same words difficult throughout their early years at school. Within each year group, the Hull model predicted between 70 and 80 per cent of the variability in spelling particular words.
"We have been able to estimate the deficit in less able children's spelling in terms of the time taken for pupils to reach the same level of proficiency with difficult words as they have attained at Year 2 (age seven) with the easy words. The time that is needed for these children to catch up is four years," said Mr Spencer.
"If all words were easy - and that means with fewer extraneous letters and greater regularity - this extra time would not be needed. Most other European countries have attempted to simplify their systems of spelling, and most have fewer literacy problems."
International measures of literacy used by the Organisation for Economic Co-operation and Development suggest that just over half of British adults have relatively poor literacy, a similar proportion to other English-speaking countries such as the US and New Zealand.
Although social and economic factors influence educational performance, the form of the written language itself is increasingly being recognised as an important factor, argues the Hull team.