If you're a sports fan, what sacrifices would you be prepared to make if they guaranteed your team a title victory? Daniel Wann, the leading psychologist specialising in sports fandoms, put this question to several hundred American baseball fans in 2011 to get an idea of their level of commitment.

Of all those he asked, more than half said they would happily give up sex, shaving, sweets and all drinks except water for at least a month in order to secure the World Series title. A third claimed that they would forgo television. A fifth were willing to wear the same underwear or to stop talking to their best friend. A small minority were prepared to stop talking completely.

When Wann asked the fans what behaviours they might 'at least minimally consider', around half said they would contemplate donating an organ if it would help their team win the championship. That's commitment - or, if you're not a sports fan, idiocy.

A bewildering 10 per cent of Wann's respondents said they might be persuaded to cut off one of their fingers, but I think we can assume that they misread the question.

As you might expect, the would-be organ donors classed themselves as highly engaged fans. 'Because they care so deeply about their team,' says Wann, 'and because being a fan is so central to their self-concept, they are willing to do almost anything for the team's success.'

Sports fans parade their allegiances in weird and wonderful ways. One Arsenal fan named his daughter Lanesra, the name of his team spelled backwards. With any luck she grew up to be a Chelsea supporter. This is not inconceivable: such expressions of fan disloyalty regularly occur in families. Wann, a passionate fan of the Chicago Cubs baseball team, grew up in a family of St Louis Cardinals fans; his sole purpose in choosing the Cubs was to stand out from his father and irritate his older brother.

Most of us are happy to express our support by cheering loudly and engaging in what psychologists call BIRGing, or 'basking in reflected glory'. To BIRG is to make a show of your association with a successful team, despite having played no role in its achievement. The term was coined in the 1970s by the psychologist Robert Cialdini, who noticed that the number of students wearing clothing bearing their university's name or logo on US campuses increased after the college football team had won. A win always makes supporters feel good, and the students wanted in on the action.

Inevitably, they were far less keen to fly their team's flag after a defeat, preferring to hide their allegiance. Psychologists have an acronym for that, too - they call it CORFing, or 'cutting off reflected failure'.

At first glance, BIRGing and CORFing seem like behavioural traits you might expect of fair-weather fans. Yet they stem from a psychological imperative we all possess: to maintain a positive sense of our own self-worth. We all want to feel good about ourselves. Being part of a group - sharing a history, a purpose, an identity - gives you that. When your team wins, you win.

The more invested you are, the bigger the emotional pay-off. As we've seen, that cuts both ways: when a team loses, die-hard fans suffer the most. In Fever Pitch, Hornby confesses that his misery at Arsenal's misfortunes could reach 'monstrous, terrifying proportions'.

Die-hard fans never abandon their club, so how do they restore their shattered self-esteem? They take the only option available to them: dig in, reaffirm their loyalty, draw ever closer to their group, sling obscenities at opposing fans and remind each other that suffering breeds resilience.

In a survey of clubs in the English Premier League between 2003 and 2013, Newson found that fans of Hull, the least successful team, reported the greatest number of social ties, a measure of close psychological kinship fans of Chelsea, one of the most successful teams, reported the fewest . Winning is important, but belonging is everything.

Michael Bond is the author of FANS: A Journey into the Psychology of Belonging