Rewarding teachers with bonuses based on their pupils' success in tests does not raise performance, a study has concluded.
The research carried out on 300 maths teachers found that incentives of between #163;3,000 and #163;9,000 did not raise class performance above levels of teachers who were not offered the cash (see box).
Over a three-year period, American academics found that money alone was not enough to help teachers increase their pupils' scores.
Despite this, a majority of teachers (66 per cent) agreed that they should receive additional payment if their students showed "outstanding achievement gains".
The study, carried out in public (state) schools in Nashville, Tennessee, comes as schools in England are likely to have more freedom in how they reward teachers.
An expected boom in academies will mean there are far more schools able to pay teachers as they wish, and Education Secretary Michael Gove has already said that he hopes to bring increased flexibility to teachers' national pay and conditions framework.
Government attempts to introduce "payment by results" in the past have been shouted down by teaching unions, which favour the current performance-management regime where teachers have to reach a wide range of performance criteria to move up the pay scale.
Professor Matthew Springer, executive director of the National Center on Performance Incentives at Vanderbilt University in Nashville, said: "We tested the most basic and foundational question related to performance incentives: does bonus pay alone improve student outcomes? - and we found that it does not."
He said the finding should "raise the level of debate to test more nuanced solutions" to reform teacher rewards and improve pupil achievement.
In the US, the Obama administration has promoted merit pay in a bid to raise attainment in its most beleaguered public schools. However, as in Britain, moves towards its implementation have been strongly opposed by the unions.
But the study threw up an anomaly. While teachers on merit pay failed to raise the scores of pupils in grades six, seven and eight above those of other teachers, there was a statistically significant rise where the same teachers also taught the fifth grade - although the improvement was not carried over into the following year.
The report said the reason for the advantage could be because the teachers had the "opportunity to increase time on mathematics at the expense of other subjects, or the fact that (they) know their students better, or something else".
Professor Springer also noted that there were no complaints from teachers about the calculation of bonuses, the payment of awards, bonuses they did or did not receive or the fairness of the process.
This in itself is significant, he said, because historically, teacher associations have opposed performance or merit-pay plans, particularly if bonuses rewarded teachers solely on their individual value-added score.
Chris Keates, general secretary of teaching union the NASUWT, said she was "unsurprised" by the US findings.
She told The TES: "People see the idea of payment by results as unfair. It doesn't take into account the variable intakes in the schools and is counterproductive to teamwork.
"There are so many factors in people's performance: to say you will deliver X because you are paying me ... Well, it's not that simple an equation.
"We have consistently argued against this and the idea that in academies 'good' teachers will get bonuses."
Beside the Point
The Project on Incentives in Teaching, called the Point experiment, took place over the school years 2007 and 2009 and involved 70 per cent of all middle-school maths teachers in Nashville's public schools.
The experiment tested no other types of incentives or systems of support for the teachers, such as professional development.
The annual bonus sums were $5,000, $10,000 or $15,000. Over the course of the experiment, Point paid out more than $1.27 million in bonuses. Overall, 33.6 per cent of the original group received bonuses, with the average bonus being approximately $10,000.
The bonuses were funded by a private donor.