Most of us have things in our youth - things we have done, things we have thought, things we have worn - of which we are not particularly proud.
But, whether our crime was one of fashion or something more serious, the chances are the motive that underpinned it was a desire to fit in with one group, or stand apart from another. Just as, in all likelihood, the mere mention of those days or threat of the emergence of photographic evidence is enough to see one break out in a cold sweat. Some things we are glad to leave buried in the past.
Certainly that would have been the easy option for Ed Husain. In fact, to many, it would have been more understandable than the course of action he took. Far from attempting to hide one of the most regrettable periods of his youth, Husain constructed his adult identity around it - or, at least, around its rejection.
As a child born of Indian parents in East London in the mid-1970s, Husain's early life is a narrative of confusion, exclusion and teenage doubts. In that, it is no different from that shared by many an insecure youth. Only Husain found an end to his search for identity in the rigid discipline of fanaticism.
The moderate preachings of his father's mosque seemed to fade to nothing next to more radical voices. He hung on the words of the Syrian radical Omar Bakri, pored over the writings of the Egyptian Islamist Sayyid Qutb and eventually graduated to Taqi Taqiuddin Al-Nabhani's militant Hizb ut Tahrir (HT). He was, he admits, seduced by an extremist Islamist creed - the call for a global caliphate, an "ummah". Fifteen years ago he was as committed to that world view as he is opposed to it today.
It took the murder of a fellow student at East London's Newham College (in 1995, the Nigerian Christian Ayotunde Obanubi was killed in one of many confrontations between non-Muslim and Muslim students) to shock Husain out of his extremist beliefs.
For the past nine months 36-year-old Husain has been based in Washington as a senior research fellow and Middle East expert at the Council for Foreign Relations.
He is the co-founder and ambassador of the UK think-tank Quilliam, advising government, monitoring the causes of radicalisation and emphasising the differences between the political ideology of Islamism and the religion of Islam.
On Wednesday the author was a key speaker in 10 Years After, a series of talks, debates and lectures examining the impact of 9/11, sponsored by the publisher Penguin and held at the RSA's London headquarters.
Husain chose to explore the roots of radicalisation and political violence - and to sound a cautionary note.
"My broad thoughts are that with the killing of Osama bin Laden and the killing of the new number two there may be complacency on the part of the West and American allies - the sense that 'we' are winning the war on terror," he says.
"I want to caution against that sense of triumphalism because Al Qaeda and the threat it posed was never about one man or any individual. It was always about an idea, a world view. And that mindset is very much vibrant today and it has every potential in the new crossroads that the Arab world is in, to be resuscitated anew. It's not over yet."
Yet for his critics a question mark will always hang over Husain's judgement. After all, this is a world view that he once swallowed whole when many of his peers were able to reject it. Once the scales had fallen, he lurched to the opposite extreme, publicly condemning the values he had once espoused with his book, The Islamist: Why I Joined Radical Islam in Britain, What I Saw Inside and Why I Left.
How, his doubters ask, can any view Husain professes to hold be taken at face value?
"People may not like it," he concedes. "But the best advocates against communism were former communists and it's sad but true that today the people who understand Islamist threats and ideologies and speak about it with the most eloquence tend to be people who've walked that pathway, not those who have studied it or see it as an area of exotic interest.
"I am not ashamed of my past, no; because when you're inside those movements you never see yourself as extreme or radical. You see yourself as being more right, having more clarity, having clearer vision than other people. There wasn't shame when I was involved and there's no shame now. But there is remorse."
He is outspoken in his view that the failure of the British government's policy in the 1990s - "that you ignore fanaticism on college campuses and throughout the wider Muslim community" - was writ large in London's 7/7 bombings in 2005.
But where once Husain suggested that groups such as HT should be made illegal, today his approach is more moderate.
He still believes that extremist views, even when held without advocating violence, provide what he calls, "the mood music to which suicide bombers dance". He views the relationship between non-violent extremism and its violent counterpart as "symbiotic", saying: "Militants have no programme without revolutionary, non-violent Islamists and every violent Islamist always comes through that background of being a revolutionary but not necessarily a militant".
But he insists there is a "third way" to address the problem as he perceives it: "The way is not to ban them, not to arrest extremists, but to provide a civil challenge to them. If these views are challenged in public by society, by the media, by university professors, then they lose their appeal. The ideology is shown to be bankrupt.
"You hold it up to the light of scrutiny rather than let it fester, which was the status quo in Britain in the 1990s."
Of the British government's controversial Prevent policy, which includes a recent initiative training university staff to recognise and report students "vulnerable to extremism", Husain is largely supportive. "I don't see it as informing," he says. "If any of us citizens of a country knew of anyone who was an advocate of paedophilia or an advocate of racism and held views that we thought might lead to some sort of action then it's a duty of citizenship to alert authorities."
Yet the question of quite where the line falls between nascent extremism and what Husain refers to as "legitimate dissent" - an inevitable part of student life and a democratic society - is a tricky one.
Husain believes that "the ideology of extremist Islamism isn't something that ought to be fought and banned per se", so much as contained through dialogue and political engagement. He hopes this might foster a sort of "Post-Islamism" - a notion he likens to the emergence of Social Democratic parties across Europe in the wake of communism.
Much, he acknowledges, hangs on the Arab Spring and the path taken in countries affected by those uprisings. He worries about what he sees as Egypt and Libya's obsession with punishing their dictators rather than "drawing up a constitution and having an election". He views such volatile conditions and policy chasms as far too obvious footholds for those keen to radicalise. All too easily, he suggests, liberal, secular pluralists can be backed into a corner by the extremists' brand of religious and political fervour that renders it impossible for opponents to "out-Islam the Islamists" and impossible to disagree and still be a "true" Muslim.
Post-9/11 there are, Husain says, "no quick fixes". His contention is that the roots of the problem that underpinned the catastrophic event "are deep and sadly growing". But, for all that, there is hope, he says. "Over the next 10 years there will be many tough questions to look at. In Europe there are issues of identity and belonging for all immigrants. For the Middle East and 'greater Middle East', including Pakistan, there are tougher issues and they're not simply economic. If they were, Bangladesh would be radicalised and it's not."
Instead, Husain says, "There are deeper issues over what it means to be Muslim in those countries.
"Islam is a religion which does not give a fixed political dogma. Islamism is a political movement. The problem is it presents the only comprehensive way of being Muslim as being politically observant too.
"But that's not the case. You can be fully Muslim and observant and pluralist in your political beliefs."
The truth of this, he says, is borne out by history, and much depends on whether it can be successfully translated into a future.