Giving schools specialist status does little if anything to improve their performance, research suggests.
About 80% of England's secondary schools now specialise
Sports colleges actually got worse compared with other schools, according to the study by Cambridge and Staffordshire university academics.
They did note a 'modest' gain of 1.5 percentage points at GCSE level for every £500 extra in funding per pupil.
The government says specialisms drive up standards and that success came from partnerships forged in the community.
The study looked at all state secondary schools in England between 1999 and 2004, taking in the percentage of students getting five or more good GCSEs and data on school expenditure.
It took into account a number of variables, to distinguish their effects on specialist schools from their effects on schools generally.
"This enables us to question whether school improvement is due to increased funding, specialist status, financial and institutional changes acting together, or is independent of these policy changes," says the preliminary report.
Schools must raise private sponsorship to bid for specialist status, then get at least £100,000 in capital funding, plus extra money per pupil.
The researchers found that increased finance was associated with a "modest" performance gain - so a fifth more per pupil, about £500, was needed to raise performance by 1.5 percentage points.
Previous studies have found no consistent relationship between resources and results in schools worldwide.
The effect of expenditure on specialist schools was not significantly different from the effect on schools in general - achieving "about the same" return in terms of exam performance as additional funding for state schools generally.
Specialist schools outperformed schools generally by around four percentage points - but statistically this was "not sufficiently robust to inform policy debate", the study found.
Specialist school performance was not significantly different for those specialising in arts, business, languages, mathematics, technology and science.
"The only major category with a statistically significant performance differential is sports, where the effect was an estimated reduction in school performance of approximately half a percentage point.
"Together, these results suggest a small positive performance effect of additional spending and that this effect does not depend on schools acquiring specialist status," the researchers conclude.
A spokesman for the Specialist Schools and Academies Trust said: "There is no doubt that specialist schools and academies continue to raise standards and provide students with opportunities to realise their full potential."
He said specialist schools performed better than non-specialist on all measures, including raw results and value added scores.
"Funding is not the only reason for this success. It is also the innovative partnerships schools build with other schools, businesses and the local community."
A spokesman for the Department for Children, Schools and Families said they had improved GCSE results.
"They also continue to have greater success at improving performance of children in deprived areas than non-specialist schools.
"Research earlier this year by Lancaster University found that specialist schools with the highest proportion of pupils eligible for free school meals have experienced by far the biggest improvement in exam results as a result of acquiring specialist status."
The chief executive of the Youth Sport Trust, Steve Grainger, said it was important to note that all specialist schools did not have the same starting point in terms of academic results.
"Our data shows that sports colleges are the fastest improving specialism, as the rate of improvement in GCSE results has been higher in specialist sports colleges when compared to other specialisms and the rise seen in the national average," he said.