They're the most photographed family in the world, but there's something missing from the public portraits of Brad Pitt and Angelina Jolie's clan - their twins, Knox and Vivienne, who have been seen only once in 406 days.

The strange dedication Brad, 46, and Ange, 34, have to keep the pair hidden is fast becoming one of Hollywood's most baffling mysteries.

‘The more they hide them away, the more people start talking,' an insider says.

In contrast to the secrecy surrounding the twins, their older kids - Maddox, eight, Pax, five, Zahara, four, and Shiloh, three - have been frequently seen in public throughout their lives. Brad and Ange have said they're intent on giving them as normal a life as possible, which is why they're seen with their older kids undertaking some of life's more mundane tasks such as grocery shopping.

The couple has refused to deprive their children of everyday experiences just because of the attention it draws, but they're careful to keep their family safe.

‘We have a high-profile family. It's a fine line,' Ange explains. ‘I don't ask for all this attention, but I'm so used to this craziness that always surrounds us.'

The Jolie-Pitts have gone as far as relocating to France in search of a more ordinary life. But the twins' lives seem far from normal. They've been kept virtual recluses and have been seen just three times since they were born 14 months ago.

The first was in a highly publicised New Idea photo shoot just after their birth; the second a glimpse of Ange breastfeeding last August, in photos taken by Brad that ran in US magazine W. They were also seen in January at Narita Airport in Japan.

In comparison, by the time Maddox, Zahara and Shiloh had reached 14 months of age, they'd been photographed a staggering 70 times in total. Pax, who was adopted when he was three, has also been seen regularly.

Hollywood sources say hiding the twins is ‘bizarre' since Brad and Ange have both spoken about their hopes for their kids to carry on their humanitarian work.

‘I want to make sure they go out into the world, as I've been fortunate enough to do,' Ange says. ‘It will make my children better people as they get older.'

Sick or stifled?

In the past 12 months the jet-set family has travelled to cities around the world - including London, Berlin, Nice, Cannes, LA and New York - but the twins are always kept under wraps.

Early this year, Vivienne and Knox were reportedly struck down by a series of viruses that caused them to be housebound. Rumours have also emerged that they're suffering serious food and environmental allergies, causing them to be confined to the home. But medical experts say that seems unlikely.

Dr John D'Arcy says there's evidence that for serious allergies small, medically controlled exposure to the allergen is the best treatment, rather than total avoidance. And in the case of less serious allergies, such as those to cats, exposure can lessen adverse reactions as kids get older.

But Dr D'Arcy adds that it's possible Brad and Ange are simply being extra careful with the twins because of the stress allergic reactions can cause.

‘Any parent will tell you watching their child suffer an unusual reaction can cause great anxiety,' he says. ‘It's quite understandable that parents will do anything to reduce the chance of their child coming in contact with something they're allergic to.'

Secret diaries

Ange has added to the mystery by saying that the twins are doing well and have recently taken their first steps. However, an intimate private journal belonging to the star, which went missing when it was shipped to the couple's chateau in France, reportedly reveals the truth about the first months of her babies' lives.

‘Ange reveals that she had a horrible bout of the baby blues about three months after the birth,' a friend says. ‘She explains what a mess she was.'

The friend says Ange's crying spells when her babies entered the world brought back memories of her stint in an LA mental institution in 2000.

Psychologist Jo Lamble says at 14 months of age and with a crowd of kids at home, there's no danger that Vivienne and Knox have suffered from being hidden from public view. But there are risks if it continues long-term.

‘In the next 12 months it will be important that they start socialising with other children,' she says. ‘If kids are kept away from social contact it can set them up for anxiety issues in
the future when they start to mix with others.'

Because of the interest in the twins, Jo believes it's absolutely vital that the babies are exposed to the spotlight gradually, as their siblings have been.

She says, ‘Imagine if they're kept under wraps until they're five then suddenly appeared
in public - it would be totally overwhelming for them.'