It has nothing to do with marksmanship. Rather it is all about going for a simple run. Well, sort of. With 30 kilogram backpacks on, the candidates are told to go for a run down a remote road. Simple enough. Except they are not told the distance. Some begin to sprint, hoping they will win if the road is short. Others pace themselves, conserving energy, in case it is a marathon.

The pressure of not knowing the distance to the finish line pushes many to breaking point. It becomes a race of psychological, not of physical superiority. "Distress tolerance," say the authors of The Power of Negative Emotion, "is important ... because it allows you to become stronger, wiser, mentally agile and, most important, happier in a more resilient, and therefore durable, way." And mental agility, they argue, must access the full spectrum of emotions; we cannot cherry-pick​ the positive ones and discard those we don't like or want to feel. Flipping the positive psychology model on its head, the authors, psychologists Todd Kashdan​ and Robert Biswas-Diener say using the power of negative emotions can, in fact, lead to greater happiness.

"We call this state wholeness," the authors say. "All psychological states have some adaptive advantage. Rather than steering you toward a single feeling state, then, we urge you to consider the usefulness of many – especially the ones we turn away from – and develop the agility to navigate every one ... "Those folk who are comfortable with being both positive and negative, and can therefore draw from the full range of human emotions – are healthiest and, often, the most successful." Their argument is firstly, that the current model doesn't work as we are seeing rising rates of anxiety, not rising rates of happiness. This is despite the proliferation of creature comforts designed to make our lives easier and more enjoyable. "Given so many amenities available to us today, we've developed a tendency to avoid discomfort," Kashdan​ and Robert Biswas-Diener argue. "We whip out our smartphones the moment we're left alone - boredom vanquished! We jockey for the fastest line on the motorway - no frustrating waits! We flip on the television when we get home from work - no other unwinding or de-stressing needed!

"What most folks don't realise is that this seemingly natural attraction to an easier life is rooted in avoidance of discomfort. People who fear rejection avoid meeting other peoplel people who fear failure don't take risks and people who fear intimacy turn to television and email when they get home from work." Avoidance of discomfort means missing life's sweeping panorama and afflicts us with a blind spot. If positivity and optimism accounts for approximately 80 per cent of success and happiness, Kashdan​ and Robert Biswas-Diener say, "tapping into the whole range of experience offers that remaining 20 per cent edge". Ironically, the blinkered pursuit of happiness can lead to a sense of loneliness as there is a gap between where we think we should be (happy) and where we actually are (perhaps happy, but probably fluctuating between a range of emotions). In fact, Kashdan​ and Biswas-Diener point out, humans tend to be "horrible" at predicting how happy we will be in the future – buying things because we anticipate they will make us feel good, but it rarely measures up.

"In a world where rejection, failure, self-doubt, hypocrisy, loss, boredom, and annoying and obnoxious people are inevitable, we, the authors, reject the notion that positivity is the only place to search for answers. "We reject the belief that being healthy is marked by a life with as little pain as possible. In fact, it's only when we are unwilling to take on the inevitable pain of life – whether it's the death of a parent, a divorce, or not getting that big promotion at work – that pain turns into something we experience as suffering. Suffering arises when we turn our backs on an escalation in emotional, physical or social discomfort. "Rather than promote more happiness, we endorse the ability to access the full range of psychological states, both positive and negative, to respond effectively to what life offers." Instead of ignoring darker emotions, we are sometimes required to draw on them. Interestingly, Kashdan​ and Robert Biswas-Diener note a number of studies and situations where negative emotions can be a resource spurring positive action:

– Students who are confused but work through their confusion perform better on subsequent tests that students who "get it" immediately. – Centenarians – people who are one hundred years old or older – find that negative feelings are associated with more physical activity. – Imposter syndrome, where we feel inadequate about our abilities and performance, can motivate personal and skill development, reflection and improvement. – Anger can stir us to defend ourselves and those we care about; guilt can indicate we're violating our personal moral code and need to adjust our actions or our code. It is a certainly a different way to consider the picture of our lives and how we can make all of it – not just the pleasant parts – work for us.

It offers a way to access the wholeness of ourselves. That said, Kashdan​ and Robert Biswas-Diener are keen to clarify that the concept is not about forcing ourselves into discomfort or enduring it with a stiff upper lip because it's "good for us". They are also careful to distinguish between circumstantial and chronic anger or anxiety (or any other 'negative' emotion). "We know pain sucks," they say. "So we want to clarify that we don't want your heart to be torn apart by frustrated goals, or by a romantic partner who sleeps with your sibling. Nor are we pushing you to hold your breath in ice-cold water without flinching. "We're just arguing that accumulating emotions that feel good now and avoiding emotions that feel unpleasant right now is not the best strategy for living well ... "Suppressing experiences is psychologically destructive because it divorces us from the full richness of life ... "To progress on your journey of personal growth, love and meaning and purpose in life, you need to become aware of all aspects of yourself, including your darker tendencies, and be agile enough to integrate them into your behavioural repertoire as needed."

In this way, they say, we start to become whole and maybe, just maybe, happier.