Nurture groups are "very effective" at building vulnerable children's confidence, but there is little evidence of improved attainment and pupils can struggle badly when they move back into mainstream classrooms.
The approach has had highly positive publicity since arriving in Scotland a number of years ago, but a new HMIE report paints a less rosy picture. Supporters criticised its conclusions, however, for failing to acknowledge sufficiently the "transformational" and "life-changing" experiences for pupils.
Nurture groups aim to provide a safe haven for struggling pupils to learn social skills and build emotional resilience, and the report - which assesses the groups wherever they exist in Scotland - finds these efforts often pay off.
The groups, which have tended to involve younger primary pupils, prove "very effective" at building resilience and confidence when targeted at P1. They are less appropriate for older children, although some do benefit.
Children experience more success, so their motivation increases. They also become more independent in their learning and in social situations.
They become better at understanding their own feelings and how their behaviour affects others. They learn to control impulsive behaviour, perhaps through breathing techniques.
The research - based on a range of sources including existing reports, questionnaires and interviews with senior staff in six authorities - also found children became better at turn-taking and sharing. They were able to follow classroom routines and social conventions, with "quiet and skilful intervention of staff" reinforcing good behaviour.
The overall picture, however, is "too variable".
Few schools were able to provide clear data showing a positive impact on attainment. Staff were advised to collect more "robust evidence" on how the children did in English and maths, and on how "wider achievements" were affected.
Nurture groups were sometimes detached from the rest of the school and suffered from ineffective sharing of information, which held children back.
"Children may experience success in the nurture group but this is not then translated to other settings such as the mainstream class," the report states. "At times, the overall learning experience for children attending nurture groups is not high enough."
Where transition to mainstream classes was not well managed, children could become "isolated and fail to develop a sense of responsibility", which "defeats the purpose of nurture groups".
The structure of nurture groups, which generally have a trained teacher and a member of staff with early years experience working with eight to 10 children, sometimes led to an "overemphasis on teacher-led activity, which can inhibit children's creativity and confidence".
The report also finds that only a few nurture groups involved parents enough.
Fair but `boring'
Irene Grant, assistant director in Scotland for the Nurture Group Network, said the report was fair for the small percentage of groups examined closely. But it was "boring and factual" even when describing good practice, failing to capture the enthusiasm of nurture groups.
The report shows the approach has been adopted by local authorities as part of larger strategies, she said. "Why would this happen if it wasn't addressing the barriers to learning and having an extremely positive impact on schools?"
There was, she added, evidence that attainment improved. In Glasgow, a 2006 study of 16 early primary nurture groups found "much better" results than in control groups. While transition to mainstream classes could present difficulties, meanwhile, it was often successful.
"Nurture groups have had a tremendous impact," she said. "That's the feedback from staff, parents and children. One head said it had totally transformed the school and how staff saw and dealt with pupils."
In Glasgow, which pioneered nurture groups on a large scale in Scotland and has 58 nurture classes, executive member for education Jonathan Findlay said: "The nurture groups in Glasgow schools have been life- changing for many children and their families.
"Our work in this field has been nationally recognised, and we look forward to continuing to do all we can to improve the life chances of some of our most vulnerable children."