A rare family archive has given historians a taste of the everyday Middle Ages, says Helen Castor

It is no exaggeration to say that biography is the cultural form of the moment. Academic concerns with oral history and life-writing - not to mention the imminent publication of the colossal New Dictionary of National Biography - are mirrored by the increasing range of popular historical writing, where "lives" of the great and good jostle on the shelves with the stories of men and women who would have found no place within the traditional confines of the biographical canon, as well as some more surprising offshoots of the genre - "biographies" of ideas, commodities and places.

More generally, the insatiable appetite for information about the private lives of public figures is matched by enthusiasm for access to the lives of "ordinary people", whether writers who come to public attention for the first time through the publication of their memoirs, or the inhabitants of the overpopulated world of reality television. Opinion may be divided on the value of some manifestations of this preoccupation with real lives, but, for the historian, the potential interest of a biographical approach is clear. A focus on individual stories offers an escape route from the neat but artificial constructs of political, social, economic or cultural history to the overwhelming complexity of lived experience.

For a historian of the Middle Ages, however, the path is not straightforward. A wealth of documentation survives from medieval England: legal and governmental records, financial accounts and property deeds, works of literature and scholarship, and narratives written by contemporary chroniclers. In none of these texts, however, can individual voices be heard speaking for themselves in private, without the constraints of technical form or the consciousness of a public readership. As a result, it is impossible to know for certain what even the most eminent members of medieval society - monarchs, nobles and prelates - thought and felt about their lives. In the case of men and women below this exalted level, evidence even of their actions can be difficult to find and harder to interpret. But a handful of correspondences - literally five collections of personal letters - has survived from 15th-century England. Of these, the greatest in scale and depth are the letters of the Pastons, a family of Norfolk landowners. The Paston archive - more than 1,000 documents, written by three generations of the family over a period of 70 years - gives unparalleled access to the private world behind the public records.

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Take the example of law court records. These are notoriously difficult to handle as historical evidence since indictments were routinely framed in such a way as to maximise the gravity of the offence. For the most part, historians have to rely on their own judgement about how far to take such allegations at face value - something that has gone a long way towards creating the popular image of the Middle Ages as a period of "sackage, carnage and wreckage" (to borrow a memorable phrase from W. C. Sellar and R. J. Yeatman's 1066 and All That ).

The Paston letters, however, offer a rare glimpse of the gap between report and reality. The Pastons brought charges against a rival landowner (Heydon) for assaulting their chaplain in a Norwich street. The chaplain was said to have been beaten so badly that he "despaired of his life". Without the family's letters, there would be no way of knowing that the brawl kicked off after a display of petty provocation from both sides worthy of Shakespeare's Capulets and Montagues, although in this instance the insult offered was not the biting of thumbs, but the chaplain's failure to doff his hat. Far from despairing of his life, he had to be held back from the fight, and then dispatched to London to keep him out of trouble.

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The correspondence also gives access to voices - especially female ones - that are rarely heard in formal documents. Of the surviving letters, more were written by Margaret Paston, who married into the family at 18 and died as its matriarch more than 40 years later, than by any other member. Her letters offer richer and more nuanced evidence of gender relationships than legal treatises or advice manuals: deference to her "right worshipful husband" was leavened by deep affection, and plain speaking when she felt it necessary. After her husband's death, her treatment of her eldest son, the new head of the family, was anything but deferential ("you are beholden to my lord of his good report of you in this country, for he reported better of you than I trow you deserve"); while at the same time she struggled to cope with the behaviour of a headstrong daughter who eloped with one of the Pastons' servants.

Evidence of this kind is so valuable that the letters have borne a heavy burden of interpretation over the years. They are such rewarding sources that there is an almost irresistible impulse to see the family as somehow archetypal of their time and class. The temptation to cast the Pastons as everyman and everywoman may explain why so few attempts have been made to write a biography of the family as individuals, despite the fact that the letters have been in print since the late 18th century.

But the idea that the Pastons might be typical of the society in which they lived is a blind alley, given how little there is with which to compare their letters. The setbacks the family suffered, for example, at the hands of another local landowner, have been used to demonstrate that the government of the mid-15th century, with which Heydon had connections, was a corrupt regime that inflicted a reign of terror across East Anglia. On closer inspection, it turns out that Heydon and the Pastons were locked in an intense family rivalry in a small corner of Norfolk, and that, whatever the wider failings of the regime, the Pastons' problems were personal in origin. Because the Paston letters survived the centuries, Heydon has been painted a thug and a bully; if the Heydon letters had done so, the Pastons would undoubtedly now be the villains of the piece.

The conclusion has to be that, as historian Ben Pimlott wrote shortly before his death this year, "a good portrait is about history, philosophy, milieu" and "asks questions as well as answering them". He was responding to the charge that biography tends to overemphasise the historical role of the individual - but, in the case of the Pastons, it is the particularity of their experiences that has not received the emphasis it deserves. They marked themselves out even among their nouveau riche peers when they embarked on an ambitious attempt to transform the family's fortunes by claiming the vast inheritance of the childless knight Sir John Fastolf.

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Over the next two decades, their battle for the Fastolf estates accentuated differences of temperament within the family. John Paston's unyielding belief in the justice of his claim, and his inability to see the world from any viewpoint but his own, made enemies of those who should have been allies; the more flexible and irrepressibly optimistic approach of his son eventually secured the possibility of a compromise settlement.

It was the personal politics of the Fastolf case, rather than overriding public loyalties, that led the Pastons onto the battlefield during the Wars of the Roses. Despite a history of service to the Yorkist King Edward IV, John Paston II and his brother fought in the Lancastrian army at the battle of Barnet in 1471 in the hope of securing their possession of the Fastolf inheritance. The choices they made as they sought to defend the family's interests amid the upheavals of civil war cannot be assumed to represent those of a wider constituency. But the more we know about how and why the Pastons acted as they did, the more we can learn, not only about a family of compelling individuals, but about the world in which they lived.

Helen Castor is a fellow in history at Sidney Sussex College, Cambridge.

Her book on the Pastons, Blood and Roses , is published by Faber and Faber this week, ?20.00.

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