Teachers hoping to get the best exam results from their pupils should consider playing them depressing music or show them a heart-wrenching, weepie film.
New research suggests that children who feel sad are more likely to do well in exams than children who are happy.
Academics from Plymouth University and from Virginia University in the US studied the effects of different moods on pupils' performance. The researchers conducted two sets of experiments, the first with 10- and 11-year-olds, and the second with 6- and 7-year-olds.
Thirty pupils in Year 6 were asked to complete a task while listening to music by Mozart and Mahler, designed to induce a sense of happiness or sadness. The happy children found it much harder to pay attention to detail than those who were made to feel sad. As a result, they performed much worse in the test.
The researchers then asked 61 Year 2 pupils to complete the same task. A third of these children were shown a clip from Walt Disney's The Jungle Book, featuring the song "Bare Necessities", designed to induce a happy mood. Another group were shown a scene from the film The Lion King, in which the young lion cub mourns his dead father. And the rest were shown a neutral scene from The Last Unicorn.
The academics found that happiness had an actively detrimental effect on pupils' performance when attention to detail was required.
Those pupils who had watched The Lion King clip outperformed both the happy pupils and those who had watched the neutral scene.
The researchers said: "Sadness indicates that something is amiss, triggering detail-oriented, analytical processing. Another possibility is that happy people avoid engaging in detail-oriented processing because doing so could disrupt their positive mood. Finally, intense positive feelings can cause widespread activation of thoughts unrelated to the task at hand."
Mick Brookes, general secretary of the National Association of Head Teachers, agreed. "It may well be that when you're sad, you take life a little bit more seriously," he said.
"If you're feeling happy, maybe you're more of a disposition to say, 'What the heck'."
But the answer is not for teachers to stand in front of the class and tell them that their pet guinea pig has died. "If you applied that logic, you would expect schools in difficult areas to be doing brilliantly," he said.
Previous research has shown that happy children outperform sad ones in tests that require creativity and flexibility.
And research from the British Psychological Society earlier this year showed that children who feel welcome and listened to at school are more likely to be academically successful. Carmel Rogers, the educational psychologist who conducted this research, said: "Students who are anxious, angry or depressed don't learn."
'A hidden cost of happiness in children', by Simone Schnall, Vikram Jaswal and Christina Rowe.