In 2016, she became the fourth chair in two years of the troubled inquiry into child sex abuse. From her early career as a social worker in Glasgow to investigating the Rotherham scandal, how has she been affected by the horrors she has witnessed?

Whenever student journalists ask me how I choose which words to quote, I tell them to pick ones that capture the interviewee’s voice. Verbal informality and vernacular bring the speaker’s personality alive on the page, whereas any line that could have been lifted from the text of an official document does not belong inside speech marks. Sitting opposite Prof Alexis Jay, however, I realise that, were I to follow my own advice this week, her voice would be almost entirely absent from her own profile.

I don’t think I have ever met anyone so fluent in the carefully dry register of judicial process. She talks like a transcript of a court document and seldom deviates from the approved vocabulary of statutory services. For example, when I ask about the Muslim men of Pakistani origin in Rotherham who were involved in child sex grooming, she refers to them as “the minority ethnic grouping identified by many victims as part of the exploitation network”.

Such discipline is very impressive, as I’m sure she doesn’t talk like this in private. The chair of the Independent Inquiry into Child Sex Abuse (IICSA) was a social worker for 31 years, and still considers it her profession. “Oh yes! That’s what I put when I fill out forms.” Born in Edinburgh and raised in a tenement by a working-class single mother, she commutes down to London from her Glasgow home. The Westminster flat where we meet is full of chocolate, biscuits and cups of tea, and when Jay laughs – a wry bubble of mirth that speaks volumes – I glimpse a much pithier, more mischievous sensibility than the one her colourless words suggest.

But this hybrid of formality and humanity was exactly what IICSA wanted when it had to appoint its fourth chair in two years, the first two having been rejected by victims’ groups as too closely connected to the establishment, the third having been overwhelmed by the scale of the job. Set up in the wake of the Jimmy Savile allegations in 2014, IICSA needed a chair sufficiently detached from the establishment to command the confidence of victims, but experienced enough to master the legal complexity of the inquiry. Jay, already a member of IICSA’s panel, was offered the post in 2016. “It was clear to me that if the government had to go out to the market, so to speak, to identify another chair, that would be potentially quite a devastating blow to the credibility of the inquiry.” Although officially retired, Jay accepted the post, in part for fear that the crisis-ridden inquiry might otherwise collapse. “I don’t wish to sound pious about this, but I regarded it as a public duty to take the job.”

Jay’s 17 months at the helm have not been entirely plain sailing, but the inquiry is no longer in peril, and last week it launched a publicity campaign for an element of its work called the Truth Project. The project is touring the country, offering anyone who suffered abuse as a child the opportunity to talk face to face with a trained facilitator. Already more than 1,800 people have signed up.

The inquiry is already struggling to fulfil its remit, given the sheer scale of its brief, so I’m curious why it has embarked on this additional ambitious exercise. “Well, it was driven by what we understood to be the need for victims and survivors to be acknowledged.” The former Tory MP Harvey Proctor, who faced lurid and false sexual allegations against him, has condemned it, objecting: “There is no veracity in the Truth Project because statements are made to it without any checks. Anyone can say anything about anyone.” Jay sighs.

“Well, that’s his view.” Does he have a point? “No. I don’t particularly wish to engage in a debate with Mr Proctor, but let me just be clear: the purpose of the Truth Project is not to test what people say. We are not there to test guilt or innocence, but to offer an opportunity to people, some of whom will never have had acknowledgement, to be heard.”

The presumption of belief has become an intensely vexed question, not least since some allegations against senior politicians have been exposed as falsehoods, so I ask if Jay’s starting principle is to believe what she hears. After a long pause, she says: “In the Truth Project we don’t challenge what people tell us about their childhood experiences. In public hearings we will take great care with complainant witnesses.” The inquiry does not refer to them as victims, or alleged victims, “because we are very careful about the terminology here”. Readers of the Guardian may think the biggest concern is victims being disbelieved – but Daily Mail readers might feel the bigger problem is false allegations. Which is it for Jay?

“I don’t know yet about the scale of false allegations. When we are due to address that, we will look in much more detail. This is an important question and I do not underestimate it at all. But, nevertheless, the secrecy that has surrounded children and their experiences of abuse is overwhelming. So I start from that point.” Is it worse to be disbelieved or falsely accused? “I couldn’t possibly make that kind of judgment. But my – and the inquiry’s – priority is the protection of children from sexual abuse.”

Jay specialised in investigating child sexual abuse throughout her career in Scotland. She became a household name in 2014 when her devastating report into child sex grooming in Rotherham revealed at least 1,400 victims. Some, including the town’s Labour MP, Sarah Champion, argue that the same anxiety about “looking racist” that deterred Rotherham’s authorities from taking action is preventing us from acknowledging that grooming rings of the type exposed in Rotherham and Rochdale are a specifically Muslim Pakistani problem. Are they?

“Hmm. Let me always be clear and differentiate between child sexual exploitation and child sexual abuse. Because, as I’m sure you’re aware, if you looked at the profile of child sexual abusers in the main, it’s white men acting alone in the home. They are the greatest number of perpetrators. The point about child sexual exploitation is that there’s an over-representation of certain minority ethnic groups, compared to numbers in the general population.”

I ask for her view on the theory that Pakistani Muslim men’s involvement in Rotherham’s grooming ring can be explained by their over-representation in the night-time economy – as taxi drivers and takeaway restaurant workers – creating an opportunity for their crimes. Jay looks doubtful.

“Ye-es, I’ve heard that theory often. But it is conjecture. I don’t know where the data is that substantiates it. Certainly in Rotherham there were a very high number of registered taxi drivers, and it may be that people were looking for an alternative source of income, in a place where there was higher-than-average unemployment. But I would never criminalise any group of people who work in whatever fashion.”

Jay is an expert in the art of defusing potentially incendiary themes. I try again. Does she share the view ofChampion and others that at the root of the problem are a minority of Pakistani Muslim men’s cultural attitudes towards white girls? “My view was and is, if ethnicity is an element of the criminality of child sexual exploitation, it has to be faced. It has to be addressed. We need to know and understand far more about this than we currently do. The big issue here is tackling the problem of demand. And that demand for sex with children is growing, worldwide.” Why does she think it’s growing? “I don’t understand it at this stage. But then I have no particular understanding of the psyche that contributes to the question you’ve just raised.”

Another element of IICSA’s work is the commission of research that will involve questioning known sex offenders about their crimes. Critics have objected that this will simply give paedophiles the attention they crave and, as we can’t believe a word they say anyway, why waste public money?

“Well, that’s their view. I do not believe our research team would be in any way naive. We have always believed it’s part of putting together a picture of what a child sex offender looks like – which must be something that the inquiry considers. We can’t look at prevention without understanding something about how the child sex offender engages.”

I ask if she ever worries that the scale of IICSA’s ambition is becoming unmanageable. She allows a dry chuckle. “Well, we’re pushing on with it. When I came into post as chair we laid out a timetable, a plan, and we have stuck to that, we are working our way through it. But there is no doubt that the more you learn, the more you want to know. And there are all sorts of areas that the inquiry will have to pursue. It’s very daunting.”

Would she describe IICSA today as a happy ship? “Well, I believe it is, oh yes.” It feels collegiate and harmonious? She looks puzzled. “Yes. I cannot say other than that. It certainly does. That’s the feedback we get internally. Why would you think otherwise?” Her confusion must be disingenuous, for anyone who follows the news would know that, under Jay, IICSA continued to suffer personnel problems. In September 2016, its senior counsel was suspended over concerns about his leadership. A month later, the BBC reported that a colleague had accused him of sexually assaulting her in a lift. He promptly resigned from IICSA, but Jay was accused of trying to cover up the controversy. Both were duly investigated, prompting excitable headlines about “inquiry investigating inquiry”, before both were cleared of any wrongdoing.

“What myself and the panel and the senior team did was entirely exonerated,” Jay offers crisply. “It was deeply unpleasant and I wish it hadn’t happened, but I don’t know what we could have done to avoid it. Everyone was glad when it was over, and there were a great many expressions of support for the inquiry. I hope that’s all well behind us now.”

I wonder what it must do to Jay to expose herself to the horrors of child sexual abuse for so many years. “I know my own limitations,” she admits. “I know I would not wish to watch videos of children being abused or listen to the sound of them being abused. I know I couldn’t control that and it could inhabit your psyche and you’d be waking up in the middle of the night. As it happens, I have [had to do that], in my past life, in other roles, and I can still see photographs in my head and you would worry about that kind of thing haunting you. But I can absolutely manage well listening to the individual talking, or reading about it. I hope I’m never immune to the impact of the experience [abused] people have and I would never want to be. I’m deeply affected by what I hear.”

People who work in her field often say they find themselves growing suspicious, even paranoid, towards men in general. “I’ve heard a lot of people say that. My optimism about humanity makes me believe every man is not capable of abuse – but I have no scientific evidence for that view.”

I had assumed that anyone doing her job would need the support of a therapist, but she hoots with laughter. “Oh no no, no no no. No, no, I have never personally felt the need for any kind of therapy. I did get quite angry in Rotherham, though.” She pauses, and allows herself a fleeting moment of emotion.

“I certainly felt a degree of pleasure and satisfaction at some of the convictions. It’s a horrible thing to say in some ways, to have enjoyed that moment. But there was one person sentenced to 35 years and that was very important. And I’m sure there were a lot of girls and young women in Rotherham who were pleased to hear that, too.”