An ordinary cheese sandwich could contain the vital ingredient to help you successfully negotiate that pay rise.

Researchers have found that people with high levels of the brain chemical serotonin are more likely to succeed in delicate negotiations affecting their own interests.

Serotonin is manufactured in the body from the amino acid tryptophan, present in several foods - cheese is a particularly good source.

A cheese sandwich before entering the boss' office could therefore give your brain that vital edge.

Researchers at the University of Cambridge who manipulated the diets of volunteers to alter their serotonin levels found when serotonin levels were low the volunteers were more likely to allow emotion to rule their heads and make decisions that harmed their long-term interests.

When the levels of serotonin were high they behaved in a more rational, level-headed fashion, putting their own material advancement ahead of the short-term satisfaction of telling their boss exactly what they thought of them.

This is one of the first studies to demonstrate the role of serotonin in regulating emotion and aggression in social decision-making. The findings help explain why some people tend to over-react to perceived unfairness, becoming angry and combative when they haven't eaten.

People with short fuses have traditionally blamed a shortage of calories and "low blood sugar" for their bad temper.

But the Cambridge psychologists said fluctuations in serotonin levels had more subtle effects. "Our results suggest serotonin plays a critical role in social decision-making by keeping aggressive social responses in check. Changes in diet and stress cause serotonin levels to fluctuate naturally, so it's important to understand how this might affect decision-making."

- INDEPENDENT