Psychology professor Laurence Alison is an expert in how to make decisions, but in the early days of his career, it was all theoretical. Then one day he took a call from “someone very senior”, who described a worrying trend: police chiefs were showing themselves unable, in critical situations, to make crucial choices. “He asked, ‘Is there anything you can do to help?’”

There was. Alison – a straight-talking, no-nonsense person – started to translate what he knew from textbooks and turn it into practical advice. “Academic work on decision-making had concentrated on studying how they’re made in theoretical settings,” he says. “But I realised we needed to move it to real-time, lives-on-the-line situations: tsunamis, earthquakes, floods, where chances were, someone was being presented with a situation where almost every choice looked dire. I knew I had something to offer that would make a difference.”

Now he and his colleague, Neil Shortland, with whom he runs training courses for military, law enforcement and political leaders around the world, have written a book that translates the wisdom they’ve honed still further, making it relatable to a wider audience. “The people we work with face tumultuous decisions on a regular basis,” Alison says. “In normal life, perhaps 1% of the decisions we make are genuinely life-changing. It’s things like whether to commit to your partner; would it be better to change career; is this the right time to have a baby? The problem is that many people are terrified of these decisions. They believe they’re bad at making critical choices. You hear them saying things like, ‘I just wish someone would tell me what I ought to do.’”

In fact, the gem at the heart of the book is that there’s almost always a decision that’s uniquely right for you – so it’s usually best to make your own decisions. It’s a question of tapping into your personal values and concentrating not on the process but on the end goal. “I’d say the biggest mistake people make when it comes to decision-making is failing to focus on the outcome,” says Alison. “They fret about making the decision, when what they ought to be doing is throwing things forward and asking themselves, ‘What do I really want to achieve here?’” Shortland agrees: “People fail to focus clearly on what matters to them. They see that an option is appealing in one sense, but they don’t think about what they need to give up to get it.”

For Alison, who teaches at Liverpool university, and Shortland, who’s based at the University of Massachusetts, acknowledging the place of regret is fundamental to effective decision-making. Fear of regretting a decision later is paralysing for some people – and that’s part of why they believe the biggest danger around decisions isn’t doing the wrong thing, it’s doing nothing. “In many ways we’re wired to want to retain the status quo, to play safe,” says Alison. “These big life decisions are unusual events in our lives. We don’t have much to compare them to, so we lack expertise – and the easy thing is to be risk-averse and stick with what we’ve got.” He calls it “decision inertia” and says it’s common in many knife-edge situations – mounting a rescue operation, for example, or choosing when to launch a military attack – where there is no perfect outcome, just “bad” or “worse”. That’s the same with some “ordinary life” decisions, too – and in those cases, what’s needed is a realisation of what’s least bad – but it’s always going to be an unpalatable judgment to have to make.

What, then, is the secret to being able to make even the trickiest of decisions? Alison and Shortland have come up with a formula with the guiding acronym “Star”. S is for situational awareness, it’s about working out what’s happening, why it’s happened and what you think is going to happen next. In their book, they tell the story of Jenny, who discovered her husband of 11 years, Rob, was having an affair with a work colleague. The discovery clearly gave Jenny a huge decision to make, about whether to stay with Rob or leave him; but first, she had to work out what was going on, both in their marriage and in the other relationship. Leaving Rob seemed like the obvious way forward, but in the end, Jenny stayed. When she unpacked the situation, she could see what had gone wrong in her marriage, but more importantly she thought it was possible to repair the damage. When you’re up against it, says Shortland, your brain is like a glass that’s already full of water. You need to let some of it out before you can think through what’s going on. You need to find yourself some space, some time, before you can unravel what’s happening.

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But time – the T of the acronym – is highly significant here, too. Because before you make a decision you need to calculate how much time there is available to make it in, and if there’s no time frame, and it’s open-ended (should I look for a new job? Do I want to move to another country?), you need to watch out that you don’t go down the doing nothing route, because you’ve got forever. You haven’t really got forever, Alison and Shortland warn: life is short, and sometimes if you choose to hang on rather than make a choice, you’re effectively making the choice anyway.

The A in Star is for adaptation. Good decision-makers are open-minded and adventurous in their headspace, and tend not to be daunted by exploring new possibilities. “Take the example of someone who gets a call out of nowhere, offering them a new job,” says Shortland. “The danger in this case is that you’ll be flattered into taking it, thinking you’ve not had to hustle for this, it’s landed in your lap, so why not take it? What you should do, though, is test it: instead of rehearsing all the reasons why it makes sense to take it, test yourself with arguments about how it’s not right.” We’re wired, he explains, to look for validation (hello, social media). But if you reassure yourself that something is right and then it turns out to be wrong, you’ll pay the price.

Finally, R is for revision, because making a decision once doesn’t necessarily mean you can’t revisit it. “The Star model is anchored around what people tend to struggle with around decision-making,” says Shortland. “We want to share the pitfalls, to describe the dangers of how your mind tends to want to go, so you can override it if that’s in your interests. We’re trying to look at decision-making as an organic process, rather than as an end in itself. Ours is a holistic approach, and it hinges on knowing what matters most to you.”

Alison and Shortland agree that some personality types find it easier than others to make decisions: they make a lot of so-called maximisers (who strive for perfection) versus satisficers, who will settle for something that’s “good enough”. The problem for maximisers is that hanging on waiting for everything to line up might mean missing an opportunity, and also, real life is rarely if ever perfect. At the root of good decision-making is the knowledge that in plumping for one option, you have to give up on other possibilities. The cooler you can be about letting them go, the more streamlined your decision-making will become.

So how good are Alison and Shortland at making their own decisions? Shortland says he was recently offered a new job and had to decide whether to go for it or not. “It was a challenge, because I had to reflect very deeply on what I really wanted,” he says. “And having written a whole book about it, it still took me five days to make my choice: self-awareness and honesty are what it’s all about and that takes time.” Alison says he still has to chew over decisions, and some are certainly more difficult than others. “My stumbling block is sometimes reacting too quickly – not taking notice of my own advice to work out whether or not I need to act at this precise moment, or if I can wait a while.”

Meanwhile, they are considering the use of artificial intelligence. “AI can play chess, it can guide fighter planes, it can spot patterns and warn us about things,” says Shortland. “But can it tell us which decisions to take right now? Could AI handle the next pandemic? We’re starting to look at the pros and cons. As with the police chiefs, the real world came calling: this is the hot topic right now, and we’re in the thick of it.”

Decision Time: How to Make the Choices Your Life Depends On by Laurence Alison and Neil Shortland is published by Vermilion at £14.99. Buy a copy from guardianbookshop.com for £13.04