Teenagers who stress about doing well in their GCSE exams are likely to get lower results than peers who remain calmer, research has found.
Researchers said pupils who worried about grades scored up to one and a half grades lower than their peers.
The study is based on a survey of 325 pupils, conducted three to four months before they took their GCSEs in 2012.
The research was carried out by Edge Hill University, the University of South Australia and the AQA exam board.
The teenagers, from the north west of England, were asked whether they agreed or disagreed with 44 statements covering three areas - exam worries, how confident they felt about dealing with their concerns and strategies they used to cope with anxiety.
Some of the statements pupils were asked if they agreed with included "I am anxious while taking exams", "If I fail an exam I am afraid I will be rated stupid by my friends" and "During exams I find myself thinking about the consequences of failing".
The findings, which are being presented to the British Educational Research Association (Bera) on Thursday, showed there was a link between worry and performing badly in exams - even after the pupils' previous achievements were taken into account.
"When prior attainment was controlled for, a higher GCSE score was predicted by lower worry and greater task-focused coping," the research paper said.
The difference between those who never worried and those that always did could be the difference between an A* and a B grade, the researchers said.
Dr Dave Putwain, from Edge Hill University, said: "There is no doubt that test anxiety, or to be more precise a high degree of worry over one's performance or the consequences of one's performance, has a detrimental effect on GCSE performance.
"Our study controlled for prior attainment and also how good students were at dealing with exam pressure and found that increased worry still predicted lower achievement."
A separate piece of research due to be presented to the Bera conference assessed the benefits of the Teach First programme, which trains high-achieving graduates to work in schools in challenging areas.
The study, by the University of London's Institute of Education, found that students taking eight GCSEs were likely to see a boost of one grade in one of their subjects if there were Teach First teachers working at their school.
The findings, based on the GCSE results of 168 secondary schools taking part in the programme between 2003-04 and 2009-10, showed that while there was no gain to the school in the first year of having Teach First teachers in post, there were benefits in the second and third years.
Teach First, introduced under Labour in 2003, has also formed part of the coalition's schools policy - Education Secretary Michael Gove has promised to quadruple the number of recruits over the course of this Parliament.