Justin Bieber, O2, London<br/> The Decemberists, Hammersmith Apollo, London

Statistically, 1,150 of the young female glowstick-wavers in Justin Bieber's audience at the 02 tonight will at some point be the victim of rape. Of those, about 34 will be pregnant as a result.

What would Justin say? The dazzle-toothed, sweep-haired heart-throb would be all compassion, wouldn't he? Not if his recent interview with Rolling Stone magazine is anything to go by. A Christian and a pro-lifer, the 17-year-old opined: "I really don't believe in abortion, it's like killing a baby!" When pressed on the subject of pregnancies resulting from rape, he added: "Um ... well, I think that's really sad, but everything happens for a reason."

Everything happens for a reason. And the reason, parents of Britain, is you. As the Smiths so presciently put it, Bieberism begins at home. You can set up all the internet porn filters you like, but good old-fashioned filth isn't what you should be worrying about: it's excessive cleanliness.

The squeaky-clean Canadian phenomenon – with the record for the most singles from a debut album on the Billboard chart, we can safely call Bieber a phenomenon – made his name via that most innocuous of channels, YouTube. His faux-DIY film clips of a capella renditions of pop hits, shared by millions, were followed by mini-documentaries screened during television ad breaks, like he was already a thing. You've got to hand it to these god-bothering neocons: they've got their game locked down tight.

As the countdown clock raises the scream volume to Heathrow runway levels, a large cage-ball sits centre-stage, as though we're about to see a human hamster. Which, in a sense, we are. The latterday Osmond starts with "Love Me", whose chorus borrows, credited, the lyrics from "Lovefool" by The Cardigans (if there's one good thing to arise from Bieber Fever it's that Nina Persson can now afford to keep making records for ever), launches into the least convincing martial arts show ever, then squeaks "I love you, Lon-din!" with all the sincerity of a Spitalfields hooker.

Fakery and fraud you expect from a manufactured pop act. What's weird about Bieber is that he isn't even technically accomplished. With his floppy-ankled gait and blatant use of lipsynch, he's no great shakes as a singer or dancer. More gifted kids than Bieber get left behind at boot camp in every season of every TV talent show.

Bieber was singled out because he is a biological freak. In every class at every school, there's one kid who hits puberty later than the rest, and is pilloried for it. In Bieber's case, it's made him a billionaire. Those never-shaved cheeks have given him a sexless prettiness at an age when most kids are out drinking and crashing cars. There have been scare stories about his voice breaking. When it does, fear not: the industry will invent Bieber 2.0.

For now, there's an hour and a half of Bieberpop to endure, consisting of bloodless R&B with every last vestige of blackness bleached out. For "Never Let You Go", he's strapped inside a steel heart and flown over our heads. For "One Less Lonely Girl", a fan is picked from the crowd, pecked on the cheek, and snapped with a toy camera by support act Willow Smith. You get the picture.

It's padded out with a turn by his backing singers Legaci, a message urging us "Don't Text Drunk", and home movies of Bieber as a toddler, singing Timberlake's "Cry Me A River". There are covers of "Wanna Be Startin' Somethin'" by "Michael Jack-sin!", whose ghost stalks the stage in the faintly insulting forms of a rubbish sparkly glove, futile crotch-grab and hopeless robotic dancing, plus Aerosmith's "Walk This Way", which omits the line "you ain't seen nothing till you're down on the muffin" because someone told him you should only sing about what you know.

From the number of crush barriers outside Hammersmith Apollo, anyone would think Justin Bieber was playing a secret show. Instead the restraints are in place to control the sedate fans of The Decemberists.

There's a feeling in the building that nobody realised how many fellow fans they had. The thoughtful, literate Oregon band used to appeal to a kind of Venn intersection between fans of alt-country and Belle & Sebastian indie schmindie, but they've gone full-blown Americana of late. With their harmonicas and pedal steel, not to mention the endorsement of REM, they'd have slotted right into the Whistle Test-led "Paisley Underground" movement of the mid-Eighties: "Calamity Song", in particular, could easily be The Long Ryders.

The Decemberists are adored by the, er, Decemberist-ists to such a degree that every song is greeted with a cheer which makes you think "OK, so this must be the hit". Indeed, it's hard not to warm to a band who can sneak phrases like "purloined in Petrograd" into an utterly trad country song.

But I also can't help thinking that their thirtysomething worshippers ought to be rushing home to check up on what their children are listening to. It might be Justin Bieber.

Next Week

Simon catches Kylie's Aphrodite tour as it touches down in Glasgow

Rock choice

The annual Teenage Cancer Trust concerts begin at the Royal Albert Hall, starring Squeeze (Tue); Biffy Clyro (Wed); Roger Daltrey performing The Who's Tommy (Thu); Beady Eye (Fri); Editors (Sat) and Tinie Tempah, above, (Sun 27). Meanwhile, Kylie Minogue launches the British leg of her Aphrodite – Les Folies tour extravaganza with two dates at the Cardiff Arena (Fri, Sat).