Kate Middleton has been married to Prince William since April 2011

Prince William was attracted to Kate Middleton when they met at the University Of St Andrews because of her gorgeous smile, according to behavioural expert Judi James.

And Kate fits in with Wills' relatives because she has the same poised demeanour as other members of the Royal Family.

'I think William was drawn to her because she has the perfect smile; bright teeth, dimples, round rosy cheeks, and in public her body language gives nothing away,' says Judi.

'Just like the Queen, whose body does not move, and unlike Princess Diana who was very easy to read.

'Usually with the Royal Family you have to watch them for a significant amount of time before they show any "leakage" as we call it.'

Since marrying 30-year-old William in April 2011, Kate, also 30, has become hugely popular with the public.

Judi says one of the reasons for this is the way she ensures she never looks miserable when she appears at official engagements.

'Kate's smile and her ability to hold it is a major asset. Normally it's impossible to hold a genuine smile for 10 minutes straight,' Judi tells the Sunday Express.

'I've tried and couldn't. She's practised a lot.'

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Anna Duff