The BBC Two documentary Transgender Kids: Who Knows Best? won’t be broadcast until 9pm this evening, but that hasn’t stopped a lot of people from forming very firm opinions about it. There has been the inevitable petition, and yesterday, the Guardian published a critical article stating that “the transgender community is ‘very scared and very worried’” by a programme that no one interviewed had, as yet, seen. The focus of that concern is a Canadian doctor called Ken Zucker, who, according to his critics, is a discredited proponent of “conversion therapy” who has prevented trans children from obtaining appropriate treatment and was fired for gross misconduct.

But in his decades-long career, Zucker supported hundred of children and adolescents with gender identity disorder (GID), some of whom went on to live happily in their birth sex and some of whom eventually had sex reassignment surgery (SRS). The allegations against him stem from an external review commissioned by his employer, the Centre for Addiction and Mental Health in Toronto (CAMH) – a review which was withdrawn from CAMH’s website after investigations showed that many claims were unsubstantiated and one key charge was demonstrably false. As the journalist Jesse Singal wrote: “it’s hard not to come to an uncomfortable, politically incorrect conclusion: Zucker’s defenders are right. This was a show trial.”

Even so, these claims continue to recycled by those who endorse a “gender affirmative” approach to trans children - where the child's assertion of their identity is accepted immediately and uncritically - and reject Zucker’s more critical practice. Such claims are very hard to reconcile with the thoughtful Zucker who appears in the BBC documentary, who exudes neither the sinister bigotry his detractors credit him with nor the bitterness that might seem reasonable in the victim of an intellectual witch hunt. However, this much is clear: he’s adamant that reflexively deferring to children who say their physical sex is wrong risks not only putting children on an inappropriate pathway to surgery, but also missing the varied issues and strains that might be behind such feelings.

In other words, children who present as trans might not simply be “girls born in boys’ bodies” and “boys born in girls’ bodies”, and appropriate treatment might involve far more subtle approaches than altering a child’s body to match their “true” gender. Highlighting this view makes Transgender Kids: Who Knows Best? an oddity in coverage of trans issues, and a valuable one. Trans people are more prominent than ever before in news, factual programming and drama; but presentation of the underlying causes is, almost without exception, extremely basic. For example, a 2015 item on trans children for the BBC’s Victoria Derbyshire show begins with images of fashion dolls overlaid with a child’s voice saying: “I didn’t want to be a boy. I feel like I’m in the wrong body.” (The charity Mermaids, which offers support for families with trans children and has criticised tonight’s documentary, recommends this video on its “resources for parents” page.)

Trans campaigners are often at pains to distinguish gender stereotypes from gender identity, and yet parents of trans children consistently refer to their child’s behaviour and tastes when they’re supplying backstories. The segue in sexism here is obvious. At one point in Transgender Children: Who Knows Best?, the father of a socially and surgically transitioned teenage transgender girl tells the documentary that he knew his family had made the right decision when he saw his child running, and thought “that’s just like a girl running” rather than “look at my son, he runs like a girl”.

Stories from the other side, though, can be similarly as disquieting: the father of a teenage girl who vehemently identified as a boy but ultimately desisted (as approximately 80% of children with GID eventually do) sounds ragged with anger as he talks about battling the girl’s wish to have short hair and dress in boyish clothes. Watching the programme, one wonders whether the care the girl received at CAMH wasn’t as much about adjusting her family’s ideas about how to be female as it was about treating her dysphoria. Context, after all, is everything. As the neuroscientist Gina Rippon puts it in the documentary, briskly dismissing beliefs in inherently sexed brains: “We live in a gendered world […] A gendered world produced a gendered brain.”

The costs of putting a child on the wrong path can be huge. We hear a lot about the dangers of suicide for trans children who don’t receive affirming treatment (although Zucker points out that his research has found suicidality among trans children is no higher than among children with depression, anxiety or ADHD), but inappropriate affirmation is no less damaging. A young woman called Lou, who started hormone treatment and had a double mastectomy before realising she didn’t want to be a man, tells the documentary that she now feels “grotesque”. The female body that had horrified her so much during puberty now seems normal to her. The flat-chested, bearded self she has now appears to her as grotesque.

It’s a deeply upsetting sequence, but an important one because it’s a reminder that there is no one simple answer here. Having to live in a body that you know is not right is surely a profound and terrible anguish: that’s true whether you think you’ve been denied vital gender-affirming treatments, or if your body has been irreversibly altered by operations you now regret. The challenge for doctors is to recognise which is which, and give children the right support. The attacks on Zucker have been effective. Around the world, clinicians now keep their concerns about gender affirmation to themselves: they know what the price of speaking out might be. But, as this documentary shows, the price of stifling that discussion can be almost unimaginable harm.