Research by the Department for Education has found that while there is evidence "specialist" teachers have a "small" positive impact on pupil outcomes at GCSE in maths, English and humanities, there is "no discernible effect" of "non-specialist" teaching at GCSE for modern foreign languages and science.
The DfE’s analysis – published in a research paper – categorises teachers as "specialist" or "non-specialist" on the basis of whether they hold a degree or other post A-level qualification in the subject they teach.
It is not possible to directly link data on individual teachers to pupil outcomes, so the analysis looks at the proportion of specialist teachers in a subject area at school level.
The DfE’s research reveals "mixed or limited findings" which "do not imply a causal link" between specialist teaching and pupils outcomes.
A "positive association" was found between specialist teaching in English and maths and GCSE attainment, but this was without controlling for other factors.
Looking at KS2 to KS4 value added – and again without controlling other factors – specialist teaching is "positively associated" with schools’ value added in English, maths and humanities. However there is "no relationship" between value added and specialist teaching in science and modern foreign languages.
When other variables are controlled for there is "no discernible effect" of non-specialist teaching on pupils’ GCSE outcomes in modern foreign languages and science.
While there is some evidence of a "positive impact" of specialist teaching on GCSE outcomes in English, maths and humanities, the DfE said this is "small in size".
"In line with most previous research, there remains limited evidence of an impact of teachers’ academic qualifications in the subjects they teach on pupil outcomes," the paper states.
The DfE admits there are limitations in its data. As well as not being able to link specific teachers to pupil outcomes, the department recognises "academic qualifications are not the only method of acquiring subject knowledge".
The definition of "specialist" used in the research does not include continuous professional development – or even native fluency in a modern foreign language – so it is possible teachers who have been defined as "non-specialist" may have equal or greater subject knowledge acquired through means other than a degree.
A DfE spokeswoman told TES: "We trust headteachers to run their schools and make the right decisions for their pupils, and the importance they place on ensuring pupils are taught by highly qualified teachers is clear.
"The latest figures show that 9 out of 10 secondary lessons are conducted by a teacher with a relevant post A-level qualification."
The study follows research by Harvard University this summer, which found that pupils who were taught by subject-specialists in English and maths saw their grades drop rather than improve.
According to the study - which was conducted at 50 elementary schools in Houston, Texas - specialist primary teachers were 6 per cent less effective than their non-specialist colleagues.
Malcolm Trobe, interim general secretary of the Association of School and College Leaders, agreed that holding a relevant degree remained important to teaching.
"All the indications are that people need to be educated to degree level in their subject or a comparable subject related to what their mainstream teaching is," he said.
"It would be extremely difficult for someone with an English degree to go on to teach maths."
However Mr Trobe said "subject pedagogy" – the understanding of how to teach a certain subject well – was just as important as having the right degree.