EVEN before Theresa May has left Number 10, people are speculating on how history will view her. Will she be treated kindly, or with derision, bafflement, or the clinical detachment she herself excels at? As the annals of Brexit are compiled, will her role be that of a rabbit caught in the headlights of the Brexit juggernaut? A principled politician who did her best in impossible circumstances? Or a victim of her own ambition, which led her to take on a role for which she lacked the necessary skills?

The answer, of course, might be a bit of all three, and a lot more besides. Right now, the only thing that can be said with confidence is that it’s far too early to deliver a verdict on her tenure as prime minister. Only after a considerable passage of time, when emotions have cooled, and events moved on, will it be possible – and fair – to assess her role with honesty. Historical evaluation requires distance and dispassion. And, while that sometimes allows appreciation of hitherto under-rated and admirable aspects of a personality, it often also results in a more damning perspective.

That said, future researchers seem unlikely to unearth anything exceptionally salacious about Mrs May in the archives. On that front she can sleep easy. Recent revelations about Martin Luther King, on the other hand, by his Pulitzer prize-winning biographer David Garrow, threaten to damage his reputation beyond repair. Almost overnight he has been transformed from charismatic political and spiritual leader to disgraceful sexual predator.

The about-turn in his popular image follows the release of tape recordings by the FBI which, on Edgar J Hoover’s orders, bugged the civil rights campaigner’s hotel rooms. In so doing, they captured King in the act of watching and offering advice while a woman was raped by a fellow preacher. On other occasions they record his involvement in profoundly abusive acts on female parishioners and others, including a prostitute who described “getting scared” during a threesome with King and a second woman.

These tapes establish King’s extramarital relationships as numbering around 40-45. Infidelity on this scale among the once-revered is not particularly new, merely confirming that, as with so many supposedly “great” men, in matters of his marriage vows King was a hypocrite of the first order. The real scandal is learning the extent of his depravity and acts of sexual coercion. Garrow admits the material on these tapes is shocking, and “poses so fundamental a challenge to his historical stature as to require the most complete and extensive historical review possible”.

Cataclysmic new information such as this is what every biographer, who thought they had nailed their subject, dreads. Yet opinion on public figures is always shifting, even without a dramatic turn of events. Already attitudes towards those such as Margaret Thatcher and Richard Nixon are turning, if not precisely in their favour then with diminished harshness. Several decades on, they are emerging as fully rounded people rather than stock characters playing their black or white, mythologised part in the nation’s story.

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When it comes to writing political biography that will endure, there’s no-one to beat Robert Caro, arguably the world’s finest practitioner. Caro, a former journalist, began his literary career with The Power Broker, a groundbreaking biography of New York legend Robert Moses. Moses was the charismatic, brilliant and venal urban planner who had politicians and building contractors eating out of his palm, while the city’s poor were royally shafted. Caro’s fascination was as much with the nature of power as with the man who wielded it so cunningly.

It is Caro’s magisterial biography of President Lyndon Johnson, however, for which he is best known. With the fifth and final volume now in progress, each book has taken years of research. In biographical terms, there’s probably never been anything to equal this venture. The result is not, as you might fear, a bureaucratic wallowing in the detail of every memo and phone call Johnson made, but a rich account of the president’s times. Out of this painstaking but imaginative reconstruction the man emerges, illuminated from every side. The facts Caro unearths never excuse Johnson’s ignoble actions, but they do help explain why he was not just a cheat and a bully but also far-sighted, courageous and revolutionary.

In his recent book, Working, a sliver of professional memoir, Caro outlines some of his thinking when embarking on biography. Anyone interested in piecing together the jigsaw puzzle that is Theresa May – or any other political heavyweight for that matter – should start here. His intellectual curiosity and tireless spadework irradiate the lives of those about whom he writes. The result is not a simple conclusion on whether somebody is essentially decent, well-motivated or corrupt, but a living, breathing portrait of their era, as well as them. Nobody in power, after all, got there by themselves. Just as each of us is composed of countless atoms, so individuals are a sum of almost infinite parts. Mrs May might take some comfort in knowing the ultimate verdict on her career should take all these elements into consideration.