Is it nature? Is it nurture? Is it simply a perverse choice that could be easily reversed with a series of cold showers, and mindful prayer? Either way, the more innocent John Barrowman fans may be shocked by the big revelation about John's private life that The Making of Me contains. Barely four minutes in, and Barrowman discloses what many could find to be an unpalatable truth about the way Barrowman rolls. I don't even know how to type it, to be honest. But maybe you know it, anyway. Yes, that's right. Barrowman is, actually ... Scottish. He might act American on television, or in interviews, but in the privacy of his own home, he suddenly starts talking like one of the Proclaimers. He makes the word “girl” go like this: “Gurrull”.

It must have a terrible effect on his ability to be gay. “You go, girlfriend!” would come out like this: “You gooooo, gurrulfriend!” It would take about 20 minutes. Barrowman claims this Scottishness comes from a combination of his parents being Scottish, and having spent the first nine years of his life in Glasgow - but many would argue that it's a deliberate choice; one that he's trying to stuff down our throats, and on prime-time TV, if you please.

The Making of Me is one of those breezy, ultimately quite ho-hum BBC documentaries that seems to get handed out to BBC stars on their way to the canteen. Stephen Fry wants to make a documentary about manic depression? John Cleese wants to hang out with lemurs? A quick phone call, and there's a camera crew, and an hour of prime-time at their disposal.

This time around - possibly to make it look as if more thought goes into these things than “celebrity doing things = ratings” - the documentary in question is part of a three-part series. Next week, Vanessa Mae will be discovering what makes her musical, and the week after, Colin Jackson finds out what makes his legs so very special and fast. But for this week, to kick off the series, John Barrowman finds out what makes him gay. His journey - for journeys are what everyone must undergo in a documentary, these days - takes him from psychologist to geneticist to his parents' house, as he asks the question, “Why am I the way I am?”

The first stop is to check that he really is gay, after all. In the 1960s Americans trying to dodge the draft would regularly pretend to be homosexual, and so the American army came up with a definitive test for gayness. To be frank, it's pretty much the test you or I would come up with: popping a putative invert into a room with a stack of pan-sexual porn, and seeing which way the horn-cookie crumbles. Barrowman scores a big 10 on the Gay-O-Meter - and so swiftly moves on to having an MRI scan of his neurological responses; just to confirm it with science. This is a process that ends with the neurologist concluding: “You have a gorgeous brain” - quite possibly proving that the neurologist is gay, too; and thus making the MRI scan one of the most effective detectors of gayness in history.

Over the next hour, Barrowman whizzes through every possible suggestion for the causes of homosexuality - genes, biological conditions in the womb, nurture. In the end, no single answer has been found - but we have learnt that if you have four sons, the chances of your fifth son being gay are 71 per cent, and we have seen a picture of Barrowman, aged 10, wearing a bikini in a beauty pageant.

“You didn't think that would make me gay?” Barrowman asks his parents - twinkling Scots, who would score highly in a “Celebrities' Parents I'd Like to Steal” contest. “We were on the QE2,” Barrowman's father replies, as if that explains everything. “We needed the money.”

We also meet a pair of 11-year-old twins in the Midwest - one of whom is almost certainly gay, and the other of whom is almost certainly straight - and learn, simply by observing, that there appears to be a gay way to bounce on a trampoline.

It's the kind of concept that could easily form the basis of a psychology paper at any half-way decent university - and is typical of this quest to find the “causes” of gayness that, ultimately, is so relaxed with itself, it doesn't really care whether it finds any answers or not.

The Making of Me, Thur, BBC One, 9pm

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