Kate Middleton's 'naughty' secret weapon for happy marriage to Prince William

Prince William and Princess Kate's marriage keeps going from strength to strength - and according to one expert, Kate's naughty trait is a major key to their success

The Prince and Princess of Wales are said to have a 'naughty' secret weapon that they use to help keep their high-profile marriage on track.

The royal couple tied the knot in Westminster Abbey in April 2011 and have gone on to welcome three children together. Given their global status, all eyes are on William and Kate when they step out in public. But behind the scenes, their relationship is incredibly relatable.

According to one expert, the couple's playful side is their secret weapon – and it's something they're also passing on to their three children. Christopher Andersen, author of 'The King', told Fox News Digital: "William once described Kate's sense of humour as 'naughty' and his own as 'dirty', and claimed that it was one of the most important things that drew them together. They've certainly cleaned up their act for the children's sake, but they can still make each other double over laughing."

William and Kate are often pictured giggling together on royal engagements - and the Wales family always seem to have fun when they step out together, especially little Louis.

The Prince first spoke about Kate's sense of humour when they sat down for their official engagement interview at the end of 2010. William told Tom Bradby: "We obviously met at university - at St Andrews we were friends for over a year first and it just sort of blossomed from then on. We just spent more time with each other, had a good giggle, had lots of fun and realised we shared the same interests and had a really good time.

"She's got a really naughty sense of humour, which kind of helps me because I've got a really dry sense of humour, so it was good fun, we had a really good laugh and things happened."

Of course, times have changed now, and the couple are proud parents to three children, but it's clear they have still got that same spark. According to Anderson, the Waleses share a "wacky, slapstick, Monty Python-meets-Benny Hill sense of humour".

And their sense of fun is something that's been passed on to Prince George, Princess Charlotte, and Prince Louis. Louis in particular always steals the show at public events with his funny antics - whether he's dancing, pretending to be a jet pilot, pulling silly faces or sticking his fingers in his ears. In fact, his parents made reference to their cheeky youngest in a thank you note at the end of the late Queen's Platinum Jubilee celebrations.

“Thank you to everyone who turned out to show their gratitude to The Queen and her inspiring 70 years of leadership. From the crowds on the Mall to communities hosting street parties up and down the country, we hope you had a weekend to remember," they wrote. "We all had an incredible time, especially Louis…"