You may be aware of the French paradox, which refers to the relatively low incidence of heart disease in France despite their rich diet.
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In rugby, we have a Welsh paradox and it's one that is particularly perplexing on this side of the Irish Sea.
Just how do they do it?
They arrive in Dublin as Grand Slam champions, a team that reached a World Cup semi-final last autumn built from a group of players that routinely loses to Irish opposition at club level.
The Six Nations era has been the most successful period in Irish rugby history, yet Wales have more to show for their efforts during the same time-frame.
Since Italy joined the tournament in 2000, the Welsh have claimed five titles with four Grand Slams, reaching the final four at the 2011 and 2019 World Cups.
Ireland remain a quarter-final team at the world's biggest tournament and, while their Championship performances have been consistent, they have claimed four Six Nations titles with two Grand Slams.
Warren Gatland's greatest trick was creating a club spirit at international level, making sure his players became better when they walked through the doors of the Vale of Glamorgan Hotel.
Sustaining that effect is new coach Wayne Pivac's biggest challenge.
After all, if the strength and success of a rugby nation was down to the performances at club level, then Ireland would be streets ahead.
In the 20 seasons since the turn of the millennium, the Irish provinces have had the better of their Welsh opponents. All four Irish teams have won the Guinness PRO14, while Leinster and Munster have six Heineken Champions Cups between them.
The Welsh regions have had to settle for five PRO14s, while the Scarlets' run to the European semi-final in 2018 was the only Welsh trip to the knock-outs since 2012.
At club level, there is an imbalance. The Irish provinces have won 58 per cent of the Irish-Welsh matches in the same period, but while they edge the international battle it is much closer.
Perhaps no fixture sums up this rivalry like the World Cup quarter-final meeting of the teams in 2011.
It was Ireland's best chance at breaking the glass ceiling, but for some reason they underestimated Wales on the back of a strong pool performance and paid the price.
Earlier this week, Brian O'Driscoll suggested Wales always used to win the big ones between the teams.
His side's 2009 Grand Slam was the exception, but in 2005 and 2019 the title was on the line between them and Wales prevailed.
In the Wellington Cake Tin nine years ago, it was the ultimate test and it was Gatland's men who progressed.
Those who played in green that day say it is their biggest regret.
Earlier this week, Ronan O'Gara described it as "a wasted opportunity", lamenting that Ireland "didn't make the mental shift necessary to appreciate what a golden opportunity it was".
Despite the relative parity of results between the teams, there has been a sense that Ireland have under-estimated Wales at too many critical moments.
During the 2000s, there was a bitter rivalry between the teams, but the 2009 Lions tour appears to have brought about an easing of hostilities and it is all much friendlier going into tomorrow's game.
Up in the coaching box sit two new faces. As the Lions' defence coach in 2013 and 2017, Andy Farrell knows the opposition well. Six of the Wales match-day 23 were Test Lions on one or both of those tours.
Across the hall, Pivac is in the familiar surrounds of the coaching booth where he watched his Scarlets team win the 2017 PRO14 final against Munster.
A year later, he was back twice to see the same side eviscerated by Leinster in the European semi-final and the league final.
He was able to get the best out of that region and a year later the WRU tried to merge them with their neighbours and rivals, The Ospreys.
Such is the way of the regional system where chaos reigns.
Over here, it's a much healthier picture at club level. Munster may have had a bad season, but they've two World Cup winners on the way next season. No Welsh region could attract or afford players of that quality.
And yet, their weakness at one level does little to undermine their international form.
Ireland may be the bookies' favourites tomorrow, but few among the home crowd at the Aviva Stadium will underestimate Wales.
Gatland and his players have proven that when they come together under the one banner, they are a force to be reckoned with. Pivac's challenge is to sustain that success without having it underpinned from below.