You can tell a lot about a boyband from the condition of their hair. In hindsight it is obvious Robbie Williams' days in the original incarnation of Take That were numbered when, circa his blossoming bromance with Oasis, he opted for a shaved bleach blond look.
Similarly, Westlife's break-up surely became inevitable the moment Kian Egan cropped his bouncy tresses and Shane Filan reduced his intake of Brylcreem – when a heartthrob can no longer be troubled to maintain a world class quiff, how can we expect them to care about their music?
In that context, recent footage of Harry Styles modelling wavy, shoulder- length curls will have tipped astute One Direction watchers off to the fact that change is afoot on planet 1D.
Indeed, with the exception of Niall Horan – still the golden-locked chipmunk who can't entirely believe he is literally the most famous 'Niall' in the history of people named Niall – the entirety of One Direction appear to have submitted to a far-reaching makeover as their fourth album arrives.
Gone are the chirpy grins and boy-next-door clobber, replaced by proper rock star threads, practiced pouts and acres of stubble. When Styles popped up on the BBC's Children In Need for a piece-to-camera last week, you could be forgiven for thinking he'd been taking presentation tips from Interpol, so gloomy was his disposition.
This switch around is reflected in the new songs – though perhaps not to quite the extent One Direction would have you believe.
Straight off, let's knock on the head the theory 1D have made their 'Springsteen' record. Those exploring 'Four' for traces of Nebraska or Darkness On The Edge In Town are likely to be underwhelmed – were it truly the intention of these Simon Cowell chaperoned 20-nothing millionaires to channel the blue collar spirit of the American everyman it is fair to say they have failed utterly.
Still girl-obsessed and squeaky of voice, they bear approximately as much resemblance to Springsteen as Jedward do to Nine Inch Nails and this misleading narrative does a disservice to what is, generally, an accomplished reasonably ambitious collection.
If there's a seventies influence on Four it is that of Springsteen's hokey cousin five-timed removed, Meatloaf. The ghost of Bat Out Of Hell haunts the swirling piano intro to recent single Steal My Girl while throughout Night Changes and Girl Almighty the group sound perpetually on the brink of the chorus of I'd Anything For Love (But I Won't Do That).
With the sound beefed up by live instrumentation, it is obvious One Direction regard Four as a transitional album. Hurtling towards their mid-20s, they are surely aware that their two dimensional reality TV pop is approaching its best by date. Thus, the fresh rawness in the vocals and even the occasional experimental flourish, typified by the growling bass riff on Ready To Run.
What a shame such innovation doesn't extend to the lyrics, which remain obsessed with young love, youthful heartache, the pain of being a besotted teenager … in short, the usual tropes and clunkers.
One Direction are clearly mindful that, for the long term health of their career, they need to grow as artists. However, Four falls short of the radical decoupling from the past such ambitions demand. It's a little rugged around the edges yet at the centre is blandly, tiresomely soppy.