Alan Gardner is a deputy editor at ESPNcricinfo. @alanroderick
Let's cut to the chase here: Joe Root is in danger of becoming the Fab Four's Ringo. The grouping of Root, Virat Kohli, Kane Williamson and Steven Smith as the leading talents of their generation has always felt a little arbitrary - what about the claims of AB de Villiers, David Warner or Babar Azam, say? - but no matter, they were locked in. However, just as Smith has taken things to a new level with his Test form, Root has experienced an appreciable dip.
Now, we're not suggesting he should turn in his membership card. Only Kohli comes close to averaging 50 in all three international formats, and Root's white-ball records are superior to that of both Williamson and Smith. But Root is, for now, the only one of the four to average below 50 in Tests - the format that, for obvious reasons, carries the most weight in the greatness debate. Root slipped below that mark in the Caribbean earlier this year, and currently sits at 48.94 - respectable but just short of exceptional.
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It is actually 50-plus years since an England batsman finished his career with a 50-plus Test average - and there is still time for Root to join what we'll call the Ken Barrington Club. But what is incontrovertible is that right now Root - England's leading run scorer on the way to World Cup success last month - is enduring his toughest spell in Test cricket since he made his debut in 2012.
Two things stand out as obstacles to the fluid accumulation of his early years: the weight of the Test captaincy, and an unsettled top order that has made Root shuttle between Nos. 3 and 4. Several of his predecessors found their batting form compromised by leading the side - notably fellow Yorkshireman Michael Vaughan, who averaged 36.02 as captain compared to 50.98 in the ranks - but after starting his tenure with an innings of 190 against South Africa, Root's stats show an increasing disparity.
When he succeeded Alastair Cook in 2017, much of the discussion was about whether the job could elevate Root's batting to another level - in the manner of his Fab Four confreres. However, it seems to have had the opposite effect, with Root averaging 42.48 as captain compared to 52.80 before.
Mark Butcher, the former England batsman who captained the team in one Test and also led Surrey on numerous occasions, believes that the demands of the job may be affecting Root's ability to consistently produce big scores.
"Even before he was captain, the top order wasn't exactly bulletproof, so he's faced that issue before, of being in early and having to rebuild," Butcher said. "So I think we can say percentage-wise the captaincy seems to have had an adverse effect. He's been doing it for too long for the jury to be out - I'm not entirely sure he's as instinctively good at captaincy as he would like to be, so I think that does take up quite a bit of mental bandwidth that was previously reserved for his batting.
"It takes an enormous amount of concentration to score runs with the consistency that he does. And if part of your brain is taken up with worrying about ten other players and all the other responsibilities you have as captain, you've used up some of your capacity."
The drop is even starker when you consider only Root's home record - an average of 59.11 falling to 45.82, around 22% lower. That said, batting in England over recent seasons has been particularly testing - the proud-seamed Dukes ball, improved drainage and floodlights (allowing for play when there might previously have been none) and faulty modern batting techniques have all been identified as reasons; and the summers of 2017, 2018, and 2019 so far, have produced three of the four lowest overall batting averages this decade.
Perhaps unsurprisingly, Root's cause has not been aided by a lack of stability in the batting line-up. With the retirement of Alastair Cook, Root is the only top-order England batsman to average over 40 (Ben Foakes averages 41.50 from five Tests, lower down), but while the team increasingly need him to shape games, match-defining innings of the kind produced by Smith - twice over - at Edgbaston have been less forthcoming.
There was a period when the chief criticism of Root's batting was a poor conversion rate; during the 2017-18 Ashes he made five half-centuries without once going on to a hundred (a bout of gastroenteritis contributed to his failure to do so in Sydney). He still managed to average 47.25, though that was dwarfed by Smith's 687 runs at 137.40. Since then, the situation has flipped somewhat - Root has three hundreds and one fifty in his last nine Tests - but his overall returns have come down. Last year, his average was 41.21; so far this year it is below 30.
While Root has commendably attempted to deal with England's top-three issue himself, moving back up a spot in the order for the start of the Ashes, the evidence suggests such tinkering has been detrimental. His career average at No. 3 is 40.57, but over the last two years that has dropped to 30.69 from 13 innings.
As well as not using to advantage his strength at No. 4, where Root averages 48.00, Butcher suggests batting at three further contributes to his mental workload. "Particularly with captains who are under a bit more pressure, which Joe has been, just having another five minutes, that little bit of extra time after you've come off the field to gather yourself and switch your brain from captaincy mode into batting mode, would only do him good," he says.
"I just think that life is being made even more difficult for him than it is already by this move [to No. 3]. It could be the making of him, it could all of a sudden flick a switch and he goes back to averaging 50 again. But I think the portents are that that is unlikely to be the case."
All that said, Root remains the standout England batsman in Tests since he took over the captaincy - scoring more runs at a higher average than any of his team-mates over the last two and a bit years. He has also been a key member of the one-day side, utilising a method that is not markedly different to his Test technique. "The hallmark of his run-scoring in Test match cricket has always been that he scores quite freely without playing big shots," Butcher says. "He ticks the scoreboard along, he's difficult to bowl at - and that's exactly the way he plays in one-day cricket."
There is one major difference, though, as Butcher notes: "Of course, in the one-day game, he's not captain."
That Root remains one of the best one-day and Test batsmen in the world is not in doubt (we'll park the T20 debate for another day). But while the Fab Four concept may be best left for timewasters on Twitter, the extent to which Root continues to trail in Smith's wake could well have a bearing on the fate of the Ashes.
With stats inputs from Shiva Jayaraman