Rory Burns and Dom Sibley, England's incumbent Test openers, have plenty in common. They were both born in Epsom, a small market town; they attended the same secondary school; they made their breakthroughs as professional cricketers at the same county; and while Burns is five years Sibley's senior, they celebrate their birthdays ten days apart from one another.
But in the last of those similarities lies an important difference. In England, the selection year usually runs from September 1 of one year to August 31 of the following year. Sibley's birthday, September 5, made him one of the oldest pupils in his year group at school; Burns', on August 26, put him among the youngest. That fact alone made Sibley twice as likely to have his name entered into an ECB database as a junior cricketer - as well as giving him a substantially better chance of attending Oxford or Cambridge University, or becoming a CEO of a major company.
"It is not the month of birth that is important per se but rather where that month falls in the selection or academic year," Tim Wigmore and Mark Williams write in their book The Best: How Elite Athletes Are Made (in Australia, the cut-off month in age-group sport tends to be January, compared to September in the UK, but the phenomenon remains the same). "Whatever month the selection year begins, the relative-age effect persists; those who are born earlier in the selection year have a far greater chance of being selected for youth teams or academies."
The logic behind the relative-age effect is intuitive. "Say my best mate and I have kids born in the same school year: one right at the start, one right at the end," David Court, the player identification lead at the ECB, explains. "One of them could be walking around kicking a football on the same day that the other one is born. That's where the gap is biggest."
"This is the paradox of the relative-age effect: it is significantly harder for later-born children to reach professional level - but, if they can make it there, they have a higher chance of reaching the peak of their sport"From The Best: How Elite Athletes Are Made
The problem is self-fulfilling. In a school playground, when the gap in physical and mental maturity between the oldest and youngest children in a year group is at its greatest, those born earlier in the selection year are more likely to be picked than their relatively young peers; in a grassroots Under-11s cricket team, the child who is powerful enough to hit boundaries will get opportunities ahead of their friend with brilliant technique but who is too small to get the ball off the square.
From there, the player who stands out based on their current performance is more likely to be spotted by scouts, offered extra coaching sessions, and to continue to improve at pace, even if their apparent superiority is rooted in physical advantage that may not necessarily extend into adulthood.
Malcolm Gladwell explained the phenomenon with reference to Canadian ice-hockey players in his 2008 book, Outliers. "The professional hockey player starts out a little bit better than his peers," he wrote. "And that little difference leads to an opportunity that makes that difference a bit bigger, and that edge in turn leads to another opportunity, which makes the initially small difference bigger still - and on and on until the hockey player is a genuine outlier."
The result is that at youth-team level, those who are born earlier in their respective selection years are over-represented. In England's last two U-19 World Cup squads, 12 players were born between September and November, and only 11 between March and August. In 2015, an article co-authored by Court in the Journal of Sports Sciences showed that out of young cricketers at the first point of identification, i.e. entry into the ECB's database, 36% are born between September and November, compared to just 16% between June and August.
While they were born five years (and, crucially, ten days) apart, the similarities between Burns and Sibley's journeys - same secondary school, same county development system - to become Test openers serve to accentuate the differences.
Sibley was a typical early-maturing, early-born child, who dominated at schoolboy level, made hundreds for England U-19s, and became the youngest double-centurion in the history of the County Championship while still studying at school. Burns, on the other hand, was a late developer with a late birthday, whose slight frame and awkward technique saw him struggle for the same sporting opportunities as Sibley at Whitgift, to the extent he changed schools for sixth form.
"Dom's parents are both quite tall, and he was always pretty tall for his age," recalls Neil Kendrick, Whitgift's master of cricket, "whereas Rory would have been the opposite. Physical strength, especially for a batter, can cause you to score a lot of runs when you're younger, but that's not to say if you don't have it that you're not going to end up being a really good player like Rory has been.
"There is a lot of proof that from an England point of view, at representative and age-group level, there is much more likelihood of you being selected at those levels if you're born between September and November or December, like Dom, than if you're like Rory with a June to August birthday."
But this advantage in the junior system does not necessarily bear out at senior international level. Court points out that two England players he classes as "super elites" - James Anderson and Ben Stokes - have birthdays in the final quarter of the selection year, in July and June respectively. This illustrates the idea of the underdog effect.
Wigmore and Williams suggest that the traits picked up by relatively young players are exactly the sort required to help athletes reach the highest level. "The very difficulties of being physically immature for their selection year - and having to struggle to out-muscle or outrun opponents and rely on other qualities if they are to compete - are ideal preparation for professional sport," they write.
"This is the paradox of the relative-age effect: it is significantly harder for later-born children to reach professional level - but, if they can make it there, they have a higher chance of reaching the peak of their sport." Their hypothesis is backed by the research: Court's 2015 paper showed that 7% of English cricketers who had made it into the system despite their late birthdays went on to play internationally, compared to only 2% of those with early birthdays.
Staggeringly, when he won his cap in 2018, Burns was England's first August-born Test debutant since the Australian-educated Darren Pattinson a decade earlier. But throughout the 2010s, there is little evidence of a relative-age effect among England Test debutants, with the same number of caps handed out to players with birthdays in each of the four quarters of the year.
"You need to look at relative age, but also their physical maturation," says Court. "You could have an early-born child who is a late maturer, or a late-born child who is early maturing, physically. What I found in my time at the FA [England's Football Association, where he worked as the performance education lead] was that if there were any Q4 [June-August birthdays] boys within the academy system, they tended to be early maturing."
There are also implications for coaches. The ECB emphasises the idea in junior development that current performance is not necessarily a good indicator of future potential, but outside of its pathways, grassroots coaches can often pay undue attention to how a young player is performing at the moment. At every level below elite, there is a trade-off between investing in a player with a higher ceiling for long-term gain, or instead chasing short-term rewards, and the balance is hard to strike.
And Burns and Sibley's cases highlight the fact that for promising young players, progress and development are not linear. Kendrick suggests that Sibley stood out as a potential future professional from early on. "Lots of things can happen, but when he was 12 or 13, I think we realised he had a good chance," he says. "I don't think you would have said that Rory was as nailed on, and he certainly wouldn't have been on the radar for England U-19s or anything like that.
"Good players can be made in lots of different ways. Some of them are prodigies younger; some aren't, and develop later. There's definitely not one thing that happens; everyone progresses differently."
Sibley, then, exemplifies the benefits of being born early in the selection year: he was physically mature for his age, especially when compared to some of his younger peers; he was given development opportunities with additional coaching which exacerbated that advantage; and he had the confidence and strength that allowed him to flourish in his early days in county cricket. After his form fell away and he suffered his first setbacks as a Surrey player, he moved to Warwickshire, and made his Test debut aged 25.
"From an England point of view, at age-group level, there is much more likelihood of you being selected if you're born between September and December, than with a June to August birthday"David Court, ECB player identification lead
Burns, on the other hand, typifies the late-developing underdog. He was small and young compared to his immediate peers, he missed out on England U-19 selection, and he did not make his County Championship debut until the age of 21. From there, he had to demonstrate his resilience and perseverance, plugging away in county cricket until belatedly getting a chance with England at 28.
Perhaps the more pertinent question is whether the relative-age effect represents a problem to be solved, or simply a fact of life. Some direct attempts to address it have been made within a sporting context: in New Zealand, rugby teams are selected by weight rather than age group at youth level, while in English football, West Bromwich Albion have held trials open only to children born between May and August in order to "unearth talented footballers who have previously been overlooked".
"It's a really tricky one," says Court. "If you see it as a problem to solve, there might be unintended consequences." In particular, he has concerns about the prospect of eliminating the underdog effect. "If some of our best players coming through are Q4s, do we then risk limiting them because they miss out on the same challenges along the journey? I'm wary of that."
The relative-age effect hints at "an inefficiency in the talent identification process", according to Wigmore and Williams, and begs the question whether young players of high potential like Burns are being lost to the game by virtue of something as random as their date of birth. For the time being, ten days can make all the difference.
Matt Roller is an assistant editor at ESPNcricinfo. He tweets at @mroller98