How could you empirically tell what makes one person happy over their lifetime?
This is a question that has been testing experts for decades but over at Harvard University, they might have just cracked it.
Some 75 years ago the Study of Adult Development at the Harvard Medical School, also known as the Grant Study, was established and has been the longest-running study on human happiness ever since.
These scientists were unusually tasked with looking at what enhanced the wellbeing of an individual rather than what deteriorated it.
Over those 75 years, they tracked the lives of 724 men, asking on a yearly basis how they were coping in every area of their lives, of which 60 are still alive and still participating.
They came from two different groups.
The first were sophomore students at Harvard in 1938, when the study first began.
The second were children from one of the poorest areas of Boston, who lived in poverty-stricken surroundings.
To obtain information about their lives, the researchers interviewed the individuals, sent them questionnaires, scanned their brains, took blood samples, studied their medical records and talked to their families.
Over time they have also chosen to integrate the men's wives and children into the project, expanding their research further.
The revolutionary research was the first of its kind in the world and it has produced some startling results.
The biggest discovery of all was that "good relationships keep us happier and healthier."
In contrast, those that were in unhappy relationships or were lonely were more likely to suffer from pain, discontent and lead unhealthy lifestyles.
Psychiatrist Robert Waldinger who is the current director of the study detailed the findings of the study in a fascinating TED Talk in January 2016.