On an ordinary Monday evening Nando’s in Whitechapel in east London is buzzing. Gaggles of girls in hijabs, groups of young men with bulging biceps, families with chubby babies, smartly dressed single women – everyone is there. They choose their chicken (boneless thighs, whole birds, spatchcocked, platters of wings, blander breasts), then its seasoning, lemon and herb, mango and lime, hot, extra hot, before anointing it with lashings of peri-peri sauce.

It is a long way from Chickenland, the small café in a grotty mining suburb of Johannesburg where Nando’s began in 1987. It was there that Robbie Brozin, one of the company’s founders, says he discovered peri-peri chicken. Spicy, healthy, different, delicious; he loved it. So he and Fernando Duarte – after whom the restaurants are named – bought the diner and gave peri-peri chicken to the world. And the world has fallen in love with it too.

Its appeal is democratic in the extreme. It cuts across boundaries of age, class, ethnicity and status. Jay-Z, Oprah Winfrey and David Beckham can be counted among its devotees. Beyoncé once spent nearly £1,500 ($1,865) in a British branch. Prince Harry and David Cameron have been spotted getting their peri-peri fix. Before South Africa’s liberation, the African National Congress’s shadow cabinet used to eat at Nando’s. Nelson Mandela was a huge fan, says Brozin.

Its less renowned diners are the most dedicated. On RateYourNandos.com fans review individual restaurants; some have critiqued hundreds. In the midst of the chaos of the riots that engulfed London and other British cities in 2011, devotees set up a “Nando’s Defence League”, begging hoodlums to leave the restaurants unscathed. Super-fans have tried to eat in every Nando’s in the world in an effort to secure the fabled “black card” which, depending what you believe, guarantees the holder free chicken for anything from a year to a lifetime.

At the heart of Nando’s success is peri-peri. In its original form the flavouring dates back to the 15th century when Portuguese colonisers in Mozambique added locally grown chillies to their lemon-grilled chicken. It spread throughout southern Africa, evolving as it went. In northern Mozambique, coconut plays a strong role. In the south, lemon and garlic are more dominant.

Brozin says they tweaked Chickenland’s peri-peri sauce for a couple of years at the start, but since then it has been more or less the same the world over. Its base is the African bird’s-eye chilli. Such is Nando’s output that 1,400 farmers in southern Africa now grow a unique variety of the plant just for the chain. Chillies are like wine, explains Sam Hirst, who oversees the farms; terroir matters. The sun, the soil, the management of the plants all influence the flavour.

The chilli is fruity and earthy, says Michelle Sparke, who is in charge of food innovation at Nando’s. It brings flavour first and then heat. As far as chillies go, the African bird’s eye is hardly one of the fiercest. At 70,000 on the Scoville scale, which measures heat, they are hotter than jalapeño or árbol chillies but far gentler than Scotch bonnets, say, or the Carolina Reaper, which comes in at a feverish 2.2m. The heat of Nando’s chillies varies between seasons but, using high-performance liquid chromatography, that of the sauce can be monitored and adjusted accordingly.

Heat, carefully calibrated, partly explains the company’s success. Evolutionarily a love of food with a kick is a good thing, argues Sarah Lohman, a food historian. Strong-tasting foods, including garlic and chilli, have anti-microbial properties. In the absence of refrigeration, heavy spicing acts as a preservative. Those of our ancestors who ate chillies probably lived longer and were thus able to have more children who would then grow up munching the same spicy food.

Biology works in Nando’s favour, too. Eating chilli peppers, which contain capsaicin, triggers the TRPVI receptor, explains Lohman, the same one that reacts to food that is thermally hot (which explains why spicy things are described as “hot”). Bodily alarm bells ring, releasing endorphins – the hormones that counteract physical pain and mental panic. It’s a bit like watching a horror movie: when your brain realises you can experience safe fear, it enjoys the sensation. “The same thing happens with spicy food. When it’s over and we realise it didn’t harm us, we want to do it again,” explains Lohman.

But humans are suspicious of unfamiliar flavours – with good reason. Strange foods can be poisonous. An adventurous palate might once have been fatal. That makes Nando’s success in those countries where it is most exotic – Britain and Australia, which between them host more than 600 of the company’s 1,000-odd restaurants – all the more striking.

In part that is because the newness of peri-peri is offset by its pairing with that most familiar of meats: chicken. Most meat-eaters like it. Few religions prohibit it. It takes seasoning well. But more than that, for Nando’s, success has bred success. People fairly reliably grow to enjoy unfamiliar flavours after trying them around 15 times, explains Lucy Cooke, an expert on children’s eating behaviour at University College London. And children are keen to conform to the tastes and habits of their peers, so even those less keen on Nando’s initially might end up visiting frequently and so teach themselves to like it. “Humans can get to like everything that’s edible,” argues Cooke.

Adolescence is a period of transition when youngsters want to be like adults but lack the confidence or experience, argues Chris Lukehurst, a food psychologist. They are learning to enjoy challenging flavours such as coffee and alcohol – and spice. Nando’s makes that easy. Peri-peri is a simpler flavour than many other options. In Britain and Australia a history of diverse immigration means that curries, from countries such as India and Thailand, are hugely popular. But those dishes are complex, with a challenging variety of notes, including bitter and sour ones. They can be unintelligible to the less experienced diner; the exact flavour and heat of a bhuna or a red Thai curry are unknown until they have been tasted. Few will be embarrassed by making the wrong choice at Nando’s. Chicken flavoured with lemon and herb peri-peri, offering “a mere hint of heat”, is unlikely to trip up the unwary. Nor is there any mistaking the nature of “extra hot: like tackling a ferociously fiery dragon”. The varying degrees of heat mean connoisseurs can work their way up to the hotter options as they become more experienced.

Nando’s owes its success to more than its food. Smart marketing helps. So too do well-designed restaurants, cheap prices and bottomless refills of fizzy drinks. But none of that is enough if the food isn’t good.