Badly behaved pupils are often given worse marks than they deserve, according to a significant report published last week.
The findings will be an important influence on moves north and south of the border to extend teacher assessment in schools. The analysis of 20 years of research has produced strong evidence that unruly boys in particular lose out under this system.
Teachers may also be biased by other factors, said Professor Wynne Harlen, who wrote the report for the Assessment and Learning Research Synthesis Group. There was some evidence that, when asked to grade pupils on individual tasks, teachers' knowledge of overall abilities influenced their judgment.
Professor Harlen, former director of the Scottish Council for Research in Education, also noted that children with special educational needs sometimes fared worse when assessed by their teacher than they did in formal tests.
However, the research message is not entirely gloomy. Provided teachers received proper training, and criteria for judging pupils were clearly defined, problems of potential bias were not insurmountable, she said.
Her analysis was based on an evaluation of 30 of the most highly regarded studies on teacher assessment worldwide since 1985.
One in 1993 in Cleveland, Ohio and New York found that children's behaviour consistently coloured judgments of ability. Since boys tended to misbehave more, they were more likely to have worse marks.
The study says this is the result of subconscious judgments by teachers, not intentional decisions to penalise troublesome pupils.
Professor Harlen's review was commissioned after an earlier study found evidence that testing demotivated learners. It offered no firm conclusions as to whether teacher assessment was a more reliable measure.
She found that teacher assessment needed clear, detailed criteria. This was most effective where teachers were involved. Policy-makers should acknowledge the shortcomings of exams and national tests when deciding whether to back it.
The report comes as the Scottish Executive is consulting over plans for a major overhaul of the 3-14 testing regime, in which ministers expect teacher assessment to play a major part. In England, the Government is considering the recommendations of the Tomlinson review which proposes that teacher assessment should play a central role in a new diploma qualification that would replace GCSEs, A-levels and vocational exams by 2014.
The report is published by the Evidence for Policy and Practice Information and Co-ordination Centre, http:.eppi.ioe.ac.uk.