A year in a dog's life is supposed to be equivalent to seven in human terms. On the way to interview Christina Aguilera, it crosses my mind that there might be a similar exponential growth rate at work for diminutive blonde pop starlets. For how else to explain that, at the grand old age of 27, Aguilera is releasing her greatest hits album? Don't most people take, well, at least a few decades to accumulate enough material? Isn't it the sort of thing ageing artists do to make themselves feel better when they're in their fifties, to remind their fans of a halcyon era before they were destroyed by the gradual onward march of time and deep-fried peanut butter sandwiches?
It doesn't help that Aguilera looks so extraordinarily youthful and tiny in the flesh (what little there is of it). When she walks into the London hotel suite where we have arranged to meet, her head is almost entirely swallowed up by a large leopard-print hat with a brim that seems to cast a shadow over her whole face. A big diamond-studded silver pendant hangs loosely round her neck. Her lipstick is a pale frosted pink and her 5ft frame has been noticeably augmented by a pair of black 6in patent stilettos. She gives a small smile and shakes my hand so gently it feels as if her wrist might fall off on to the patterned carpet. The whole impression is that of a little girl playing dress-up in her mother's wardrobe.
She doesn't really go for the canine seven-year theory when I put it to her. 'Yeah, it's crazy,' she says, looking at me slightly oddly. 'It's been quite the journey.'
Certainly Aguilera seems to have been born with the kind of precocity gene that makes Shirley Temple look like a late developer. She was performing in local talent shows at the age of six. Her first television appearance, as a contestant on the long-running US series Star Search, came two years later. When she was 12, she landed a presenting job on the Disney Channel's New Mickey Mouse Club (her contemporaries included Justin Timberlake, the actor Ryan Gosling, and fellow blonde pop poppet Britney Spears). She was 17 when she released her first single, 'Genie in a Bottle', which went straight to the top of the American charts and stayed there for five weeks. That means that although she is still three years shy of her 30th birthday, she has nonetheless racked up 19 years' worth of professional experience.
Oh, and she has an 11-month-old son.
'Yeah,' she says, with a small, high-pitched giggle. 'People forget that I've been in this business since I was six years old, you know, performing and getting gigs and doing shows. But really it's something that I wouldn't see being any other way. At a really young age, I just gravitated towards performing and singing: it was what I was passionate about. Some people gravitate towards their Barbies; I gravitated towards the microphone. That was how it was.'
It would be a mistake to underestimate her. For all her girlish demeanour and delicate physique, Aguilera possesses an extremely powerful and technically skilled soprano voice that has catapulted her into the modern pantheon of diva stardom. She has been compared to Whitney Houston and Mariah Carey. Celine Dion has described her as 'probably the best vocalist in the world'. Her vocal range is believed to span four octaves and reaches a high E - an interval of a major third above the climactic high C of Madama Butterfly. She has won five Grammys and is one of the most successful recording artists of the decade, having sold more than 37m records. Her ballads, with such titles as 'Beautiful' and 'The Voice Within', have become classics of the genre, full of uplifting lyrics, tumescent cello chords and the endless potential to be mangled by contestants on The X Factor
She is refreshingly dismissive about all of this. Given that she lives in an $11m mansion in Beverly Hills, I half expect her to be terribly LA and to regale me with an extensive run-down of the vocal exercises she does each morning before her yogic salutation to the sun and her dairy-free breakfast. Instead she is almost absurdly matter-of-fact about it.
'Yeah, well, you know,' she says, wrinkling her nose. 'I don't try and restrict myself too much. I honestly don't even know what my range is. Because I've never been classically trained - or trained at all, actually - so I never know what I am. I do a lot of cardio exercise because it just helps the breathing and the pipes and all that. But if you're prepared, those things [like hitting the high E] aren't difficult at all.'
She says there was no one moment of realisation about her voice - it simply crept up on her and she never thought it was anything particularly special. For everyone else, though, it must have been quite strange hearing this belting, big noise coming out of a six-year-old's mouth, especially as there was no family history of singing (she thinks a grandmother of hers might once have been in a choir). Pop folklore has it that when Aguilera started competing in local talent shows in her hometown of Pittsburgh, the other contestants would pull out if they heard they were up against her.
'I would listen to my Sound of Music soundtrack and take it up to my bedroom, to the window, and sing out almost like I was pretending I had an audience,' she says. 'It was just something that I did and I dreamt about. By the time I reached six years old, I was pushing my mom to let me get on stage and perform because that's what I loved to do.'
Aguilera's mother and father were not typical stage parents. She was born in Staten Island, New York to an Ecuadorian father, Fausto, who was a sergeant in the US Army. Her mother Shelly was a Spanish language teacher and Aguilera had an itinerant childhood, moving from one military base to the next. Her father was verbally and physically abusive, and her parents divorced when she was six. Aguilera, her mother and her younger sister, Rachel, moved to their grandmother's home in a suburb of Pittsburgh.
She says that her singing initially came about as a sort of avoidance tactic. 'As a very young girl there was a lot of domestic violence in the home, so I think I just gravitated towards music as an escape.'
The experience left an indelible imprint. It gave Aguilera her drive and a determination that other women should not have to suffer the same fate - to this day she donates clothes and money to a domestic-violence shelter in Pittsburgh. But it also meant that, over the years, she built up a ferocious internal defence mechanism. Although she is very punctual and polite and answers all the questions I put to her, she does so with extreme guardedness. On several occasions, her eyes acquire a sheen of impenetrability and she morphs into cliché rather than expressing what she might actually feel. She talks about things being 'blessings', about how her songs are like 'my children' and about how 'time flies when you're having fun'.
It is not, I think, that she wishes to be difficult or to give nothing away but more that it takes an awful lot for her to trust other people. When I ask her about this, it prompts a rare moment of hesitation: 'I definitely, you know, uhhh... have my moments of being shy and a little more introverted.'
Partly this stems from her relationship with her father and partly, she says, it has to do with having been ostracised by other children who did not understand her. While Aguilera was performing in spangly dresses on prime time, her peers were mostly trying out for the ice hockey team. She was bullied at school. The tyres on the family car were slashed, forcing them to move home.
'I would get a lot of cold shoulders because there was just no way they could relate to what I loved to do. You know, it's not really normal for a child to just want to be in front of the camera and on stage. It's not something that all kids want to do - they want to play in the playground. All I can think...' she pauses, scanning the room vaguely with her eyes, 'that was my form of release without my even knowing it at that young age. You know, it was hard for me to relate to other kids because I didn't have the same interests. I was even more the oddball, I felt, because of that.'
When, aged 12, she started doing the New Mickey Mouse Club, she remembers feeling an overwhelming sense of relief. 'It was the first time I was with a bunch of other kids who loved doing the same thing and were as passionate about it [as I was]. So it was really exciting for me to almost feel I'd found my kind.'
Her mother was, she claims, supportive but not remotely pushy. 'I was pushing her to get me involved!' Aguilera says. 'She was just a mom that was proud of her daughter.'
Like most of the best female divas, Aguilera has through the years acquired a reputation for being somewhat high maintenance. Her concert rider demands include L'Occitane vanilla-scented candles and bottles of Fiji mineral water served at room temperature. Sure enough, before Aguilera even enters the room, I notice that someone has thoughtfully left precisely the right kind of candle and water on the coffee table. Yet in person she does not seem remotely prima donna-ish, sitting on the sofa with her legs folded up to one side and apologising if she seems a bit hazy from jet lag. Does the demanding reputation come from a desire to control her surroundings, to prevent everything from slipping back into chaos?
'Yeah, I think that might have something to do with it, because I was raised in a very chaotic environment. We were always moving from one place to another, my father being in the military, and you know, my mother leaving my father, going back and forth. Nothing was ever quite stable or secure for me. So that might definitely have something to do with liking things done in order, in control and my way.' She laughs. 'I'm a huge organisation freak. Everything is labelled and specified and there's a place for it.'
Her career path has been plotted with the same careful determination. Originally marketed as a bubble-gum pop princess (the video to 'Genie in a Bottle' featured the obligatory teen-girl-dancing-on-a-shiny-red-car interlude), she became unhappy with her lack of creative input and sacked her manager.
In 2002, she released her second album, Stripped, to the sound of a thousand jaws hitting the floor. The album caused an outcry over its unapologetically adult content. The video for 'Dirrty' (sample lyric: 'sweat until my clothes come off') featured Aguilera in a pair of crotchless leather chaps dancing provocatively in a boxing ring among mud wrestlers and contortionists.
'It was my coming of age - sort of being free of all inhibitions,' she says now, smiling fondly as if remembering a kindly great aunt. 'Part of what I love about being an artist is being able to spark people's opinions, you know, give them something to talk about... to be able to spark conversation as to why, you know, why is it OK for a guy to be sexually open, but not a female? Why is it OK for a man to feel empowered from it, but not a woman?'
Interestingly, Aguilera says that if she had not been able to wrest back creative control, 'I don't think I'd be around. I really don't think that anything that I would have been able to do would have been honest and sincere, and I think the public would have felt that.'
But given her previous reputation as an unthreatening teenybopper, you can also understand why some of Aguilera's critics at the time claimed that her overtly sexualised new image was not suitable for her legion of younger fans. Does that bother her?
'You know, I think it's a matter of parenting,' she says. 'I think more parents should talk to their kids. I think a lot of parents are afraid to, and I think that sometimes makes a child grow up and experiment on their own, explore things on their own - and then you get the parents that don't really set limitations or guidelines.
'I was 21 years old when I released "Dirrty". I'm not there to parent anybody's children. I'm an artist and I'm expressing myself. If you're eight or nine years old you shouldn't even really be watching MTV because there's explicit material all over the place.'
What did her own mother make of the furore? Aguilera smiles. 'She says: "She gets it from somewhere, you know."'
In the end, the gamble turned out to be spectacularly worthwhile: Aguilera's professional reputation has gone from strength to strength. In 2006 she released Back to Basics, a critically acclaimed album infused with the big band sounds of the Thirties and Forties and influenced by the legendary jazz and blues singers Etta James and Billie Holiday. It was the kind of career that Britney Spears was meant to have but never quite achieved, instead going through a very public breakdown in the latter part of 2007. Of course Spears never had Aguilera's vocal ability, but I wonder if Aguilera ever thinks she could have suffered an equally brutal disintegration had she not been brave enough to follow her own instinct?
'No, not at all,' she says baldly. 'I think it's been very obvious through the years how different we are. Maybe at first, with her and me being the two young female artists at the time, the most successful acts at the time, then of course there were going to be comparisons - but I think over the years it's been very obvious that, you know, we've grown, and how different we really are. But I really can't comment further on that because I just don't know. I can't judge.'
In any case, the leather chaps went into storage when Aguilera married music producer Jordan Bratman in a Christian Lacroix dress in 2005. According to one paper, Aguilera even removed each of her 12 piercings 'as a mark of respect', and if that's not true love, I don't know what is. The couple have a son, Max Liron (which translates from its combination of Latin and Hebrew as 'our greatest song'), who will turn one in January.
I ask her whether motherhood has changed how she feels about her own father - although they met briefly in 2000, she is no longer in contact with him. 'Um. No. Not really. I don't really think it has anything to do with him, to be honest. I mean, I can understand your point but, ah, I think that Jordan is such an incredible father and, you know, whether or not anything would ever happen to us, I know that he would still always be very much a part of Max's life. And I can't really say the same for my own father. I think if he might have been different, it would have been a different situation. But I think you reap what you sow.' She sits back on the sofa with a distant smile and waits patiently for the next question - Aguilera is one of the few people I've met who doesn't feel the need to fill a silence with chatter.
Her estrangement from her father made Aguilera mistrustful of men for a long time. She prides herself on never having been in a violent relationship.
'I think just being the person I was, really, I took that in a very defiant way: that I was never going to let a man ever do that to me. I was never going to feel helpless to a man. I was never going to be in that position. And I made that decision really early in life and I really stuck to that. Plus, I think
I never went through that completely boy-crazy phase because I was always so goal oriented in what I wanted to do as a performer and I think knowing that... that was my form of independence.'
She describes Bratman as her 'rock' and says that, if anything, married life has got better since Max's birth. 'This little one is a constant reminder of your love for each other. It's, um, also a work in progress because you're constantly growing as people. It can be very challenging. It can be incredible and exciting. I got lucky. I have a great husband - he's really supportive and, ah, I look forward to the next one.'
The next child, I say, not the next husband? Aguilera looks momentarily startled and then bursts out laughing - a genuine, full-throttle fit of the giggles. 'Exactly. Right. That's a good one.'
It is, I think, the only moment during the interview when Aguilera forgets to be guarded. For those few brief seconds, all the wariness and circumspection vanishes and she is utterly and completely herself. She laughs without thinking why she shouldn't. It is only a tiny glimpse of the real her, but I'm glad that, finally, I get to see it.