Expectations and rewards are the key determinants in the happiness equation developed at University College London. Expectations enjoyed an edge over rewards.

Overall wealth accumulated during the experiment was not a good predictor of happiness.

Lead author of the study, Dr Robb Rutledge (UCL Wellcome Trust Centre for Neuroimaging and the new Max Planck UCL Centre for Computational Psychiatry and Ageing), said: "We expected to see that recent rewards would affect moment-to-moment happiness but were surprised to find just how important expectations are in determining happiness.

"In real-world situations, the rewards associated with life decisions such as starting a new job or getting married are often not realised for a long time, and our results suggest expectations related to these decisions, good and bad, have a big effect on happiness."

For the study, 26 subjects completed a decision-making task in which their choices led to monetary gains and losses. They were repeatedly asked to answer the question 'how happy are you right now?'

Their neural activity was measured during the task using functional MRI and from this data, scientists built a computational model in which self-reported happiness was related to recent rewards and expectations.

The model was then tested on 18,420 participants in the game 'What makes me happy?' in a smartphone app developed at UCL called 'The Great Brain Experiment'.

There was no money to be won, just points. But to their surprise, scientists found that the same equation derived earlier could be used to predict how happy subjects would be while they played the game.

This is evidence, they say, of the value of this approach for studying human well-being on a large scale.

The equation indeed is not just an intellectual exercise but has practical applications, the scientists contend. By quantifying subjective states of happiness and depression, doctors can be made to understand mood disorders, to know how mood is determined by life events and circumstances, and how that differs in people suffering from mood disorders. This will hopefully lead to more effective treatments.

The equation could also assist governments by providing quantitative insight into what the collected information on some well-being policy measures may mean. This is especially relevant to the UK following the launch of the National Wellbeing Programme in 2010 and subsequent annual reports by the Office for National Statistics on 'Measuring National Wellbeing'.

The study was published in the Proceedings of the National Academy of Sciences.