Which of these couples will last the distance? Photo / Getty Images
With a royal wedding looming, it's hard not to compare the two couples the world will be watching.
Kate and William and Meghan and Harry are very different couples in very different relationships, reports Tracey Cox for the Daily Mail.
Kate and William took the traditional path: they were college sweethearts before they got married and had children, so have grown up together.
Meghan and Harry met in their 30s with past relationships and life experiences in tow.
Kate might be a "commoner", but she was born into an upper-middle-class family with British parents and relatives who played host to British royalty 'as long ago as 1926'.
Meghan's mixed race, American, Catholic, divorced, an older woman, an actress and about to marry an English Prince.
Traditional royalists threw their hands in the air when Britain's most eligible bachelor chose her to be his wife even if the rest of us cheered (Vive La Difference!)
Kate respectfully toed the line right from the beginning, Meghan seems to be ripping up the royal protocol book as she goes along.
The big question is which approach and relationship is going to survive the intense pressure of being royal long term?
Of course, there's another important factor which hugely influences all their lives: William is highly likely to be King and Kate most probably his Queen.
The responsibility on their relationship is far greater and Meghan and Harry will have far more freedom.
Personally, I'm batting for both couples but there are strengths and weaknesses to each relationship style.
Which one is more likely to last the distance: traditional or modern?
WHY THEY'LL LAST THE DISTANCE
She's got the right background
She's educated (a degree in Art History) but not particularly ambitious career-wise – possibly because she met William early in her life and knew that if they married, she'd have no chance to further it anyway.
William is heir to the throne after Prince Charles and needs a wife who can devote herself entirely to him and royal obligations; Kate's clearly prepared to do this.
They're both crystal clear on what's expected of each other which means no misunderstandings about their roles within the relationship.
He gives her priority
William mostly toes the line but speaks up when it comes to family. He continues to break protocol by travelling with Kate and the children (multiple heirs to the throne shouldn't travel together in case there's an accident).
He has a strong commitment to his personal relationship with his family as well as respect for his royal role.
They took their time to get to know each other
Kate and William met in college and didn't get engaged until five years after they graduated.
Couples who get engaged after a longer engagement tend to last longer than couples who get engaged while in the first flush of romance.
The pair also broke up for a few months, then reunited, showing they're capable of healing old wounds and moving forward without dwelling on past slights.
They accept what's expected of them
Compare the engagement interviews of the two couples and Kate and William seem formal and unaffectionate.
They sat apart and rarely touched – the most we saw was the odd pat on a knee – and angled their bodies towards the interviewer not each other.
But this was because they were following the unwritten protocol set by Queen and Prince Phillip who virtually never indulge in PDA's.
Kate treads carefully and defers to William, lets him do the leading and talking – all of which is expected of a future King of England.
She dresses the part
Some say her clothes are matronly and way too old for her (Kate's one year younger than Meghan but appears older probably because of her dress sense) but her style is classic, timeless and very royal.
Besides, she can't be that boring because she created the 'Duchess Effect': when Kate wears something it almost instantly sells out.
They have history and are very similar
Couples who get together young have history: they know so much about each other, they don't have to explain themselves.
They're both English and have a lot of the same friends as well as shared cultural references and memories.
All of this is incredibly important in long-term relationships.
The couple that play together, stay together
They might not be publically physically affectionate but there's lots of eye contact and clear affection between Kate and William.
They're smiley and often photographed having a good laugh together.
This suggests they prioritise having fun and this is an accurate predictor of high relationship satisfaction and an important part of a successful relationship.
They clearly share the same sense of humour, they're both sporty, up for a laugh, and competitive in a good way.
They like doing the same things as well: Kate admits they have 'date nights' where they stay in, cook and watch Game of Thrones.
She's a good breeder
The couple have a boy, a girl and a third child on the way: they've created the perfect family.
POSSIBLE PROBLEM AREAS
Meeting young has its drawbacks
Unlike Harry who's played the field and Meghan who's been married before, Kate and William have been together since their teens, which means they've possibly missed out on a lot of partying.
This can lead to feelings of having missed out on youth.
Change can feel more threatening
Another problem with getting together young is that if one person starts to grow or change in another direction, it can feel more destabilising.
You're much more likely to share the same opinions if you've grown up together and shared the same experiences.
You're not used to thinking differently so if one of you does, the person who remains unchanged feels duped – What's going on? You'all-encompassingere?
The person who is evolving can feel held back.
Change is nothing to be frightened of, but it can feel more threatening in this scenario.
William's commitments are all encompassing
He tries his best to accommodate his family but the cruel reality is, when William does become King his workload will become cripplingly exhaustive.
In many instances, he will have to put the monarchy before his personal relationship with Kate.
We all watched The Crown and saw the effect that had on Queen Elizabeth's marriage.
WHY THEY'LL LAST THE DISTANCE
They were introduced by a mutual friend
Research proves friends are a much better judge of what suits us relationship wise – way better than parents, relatives, or dating apps.
If friends think they'd make a good match, they're probably right!
They kept their relationship secret for six months
Friends and family influence us whether we like it or not, so it's very wise to make your own mind up about a future partner before letting everyone else give their opinion.
Imagine the entire world having their say on your choice!
Knowing this would happen, Meghan and Harry very sensibly got to know each other by laying low and simply hanging out – for almost a year.
This meant the relationship wasn't put under intense scrutiny when it might have buckled under the pressure (as was the case with Chelsea and Cressida).
She'll cope well with the strains of being a royal
As an activist, Meghan's capable of holding her own and standing her ground. As a celebrity, she's used to being in front of the cameras and being the centre of attention.
As we saw with Diana, it's not easy being heard in the royal household and strong will is essential; she's also unlikely to be completely over-whelmed by constantly being under a frequently harsh world spotlight.
During the engagement interview, Meghan led and carried it: Harry was happy playing second fiddle.
She's strong enough to control 'bad boy' Harry
At 11, Meghan successfully campaigned to get a company to change what she saw as a sexist TV ad – this is no shrinking violet.
She's feisty, opinionated, charismatic and dominant - she has publicly voiced her 'pride in being a strong, confident, mixed race woman'.
Her Dad is Caucasian and mother is African American: she says she's 'half black and half white' and she voices 'my pride in being a strong, confident, mixed race woman'.
Harry's a nice guy but he is a former party animal - I suspect he needs someone who's going to keep him in line.
She's a humanitarian like Diana
It's no secret the boys adored their mother and Meghan is an activist with an impressive humanitarian work history.
She works with the UN, has a degree in theatre and international studies and identifies as a feminist.
While she is giving up acting once married, she also makes it clear she intends to direct all that energy to humanitarian work.
Meghan's helping Harry carry the torch for Diana who almost single-handedly changed world attitudes towards things like AIDS.
It's no bad thing that Meghan is feisty and a rule-breaker –one reason why Harry he probably went for her in the first place.
Remember, Harry and William are the sons of Princess Diana who broke a lot of rules.
She's emotionally open
The couple sat shoulder to shoulder during their engagement interview, holding hands with fingers fully entwined – the trademark hand-hold of a couple in the heady 'honeymoon' period.
It's obvious they're besotted with each other but even more obvious that there's a deep connection: Meghan sees Harry as Harry, not just a Prince.
Harry, Kate and William are strong advocates of mental health and encourage everyone to open up and talk about real issues.
In true American fashion, Meghan's not afraid to talk about hers.
She's a homebody
The pair effectively hid away for months, avoiding the public eye until they knew each other well.
As anyone in a long-term relationship knows, the couple who can happily stay in together, with just the telly and each other for stimulation, work well.
He cooks a mean roast chicken, apparently, and she seems to equally enjoy low key hanging out.
They have common goals
Harry wants to 'modernise the monarchy' and he couldn't have picked a better person to do it with.
This suggests they think the same and have the same common goals which is crucial for high relationship satisfaction.
He's letting her be her
She wore faded blue jeans with rips and a white shirt to her first ever public appearance with Harry – Twitter trolls condemned her but he didn't care.
He chose someone who doesn't conform to usual royal 'standards' and seems happy that she doesn't.
She wants to fit in
Pictured together at the Royal Foundation Forum, we saw our first glimpse of the two couples together as a foursome.
Meghan was clearly mirroring Kate – deferring to her as the expert in how to behave – both with crossed ankles and hands in their lap.
She's individual but happy to conform to fit in: another quality which will stand her in good stead in royal life.
As the relationship between Harry and Meghan progresses, the dynamic is changing.
She's acting more adoring and more deferential to Harry – she's 'bigging him up'. He's acting more like a future husband than a besotted fan and becoming more confident.
Rather than her being threatened by losing control, Meghan's embracing it: she's flexible and willing to change to suit the relationship.
They met later in life
This can mean the couple are more committed – they know what's out there and know when they have a good thing in front of them.
The older you are when you meet, the more formed you are as individuals: you bring life lessons and self-knowledge into the relationship.
Meghan's been married before so isn't naïve about relationships: she knows they can fail but it hasn't put her off giving it another go.
He's protective
Harry's made it clear right from the start that Meghan is his choice and it's non-negotiable.
When news of their engagement was followed by a 'wave of abuse and harassment', he responded quickly with a strongly worded statement saying he was deeply disappointed that he wasn't able to protect her and horrified at the sexist, racist comments.
It was one of the most powerfully worded public defences of privacy any member of the royal family has ever issued.
Feeling protected by your partner and that they're on your side, is an important quality for future happiness.
POTENTIAL PROBLEM AREAS
She's giving up a lot
Unlike Kate, Meghan was a highly successful career girl.
She worked hard to get where she got, she's used to lots of attention, calling the shots and being the 'star'.
She's better equipped to deal with the media attention and used to the world's eyes upon her but she will be reined in and might not enjoy the loss of freedom.
She's divorced
The fact that she's divorced means she has no intention of staying married unless it's working.
Studies show couples who are as committed to the institution of marriage as they are to each other last longer than those who aren't.
Getting together older means you're more set in your ways
Both are used to being independent, doing things their way and not having to answer to anyone so it could be more difficult for them to make shared decisions.
There's also evidence that – contrary to popular opinion - people who've 'sown their wild oats' find it harder to settle down than those who haven't.
It's likely they'll have children quickly
Meghan's 36: not over the hill fertility wise but no spring chicken either.
They'll probably want to get a move on with having a family which means less time together just as a couple.
Studies show couples who have a strong foundation of time together before kids coming along are better able to cope with the stress of parenting.
Neither – and both.
Traditional style relationships work well for certain people and not others; ditto more modern style couplings.
As close as they are, Harry and William are very different men who need different things from their partner - and that's exactly what they've got.
Meghan is a brilliant choice for Harry and both are suited to a modern marriage; Kate is equally as perfect for William who both fit the traditional mould.
That's what makes them such a dynamic quartet.
Like the rest of us, their relationships will have their ups and downs: just because they're royals, doesn't mean they won't be affected by the pressures of life and marriage.
But my money's on both of them lasting the distance.
Washington Post: The mindset should be - less fuss, more fun with company.