When Perth Scorchers meet Sydney Sixers in the final of this year's Big Bash League, it is a safe assumption that, unless the wicket looks unusually bowler-friendly, both teams will want to chase. That has been the trend all summer long in the BBL. Twenty-seven of the 34 teams that have won the toss have opted to bowl. The seven who have decided to bat have all regretted it: they have lost every time.

Ian Chappell's aphorism - when you win the toss, you bat first nine times out of ten; the tenth time you think about it and bat first anyway - has been inverted. Yet it held true in Australian T20 cricket for many years. In the first season of the BBL, in 2011-12, just three times out of 31 did teams who won the toss opt to chase, a similar figure to the pre-BBL days in Australia. But ever since, chasing has becoming increasingly popular. Just over half of all teams opted to chase in the previous two BBL seasons; 79% have done so this year.

The penchant for chasing in Australia is a manifestation of a much larger trend. Until the end of 2013, 59% of teams chose to bat first, according to a database from cricket statistician Ric Finlay. Yet in recent years there has been a distinct shift towards chasing: from 2014 until the end of 2016, just 39% of teams who won the toss opted to bat first in matches. Chasing teams get a small but significant advantage: in the 2014-16 period, they won 5% more matches than those batting first.

The bat-second preference (as of January 25, 2017)
Year Percentage of teams inserting the opposition
2003 45.83
2004 51.02
2005 40.26
2006 28.42
2007 44.83
2008 42.08
2009 35.98
2010 34.81
2011 45.05
2012 43.45
2013 48.50
2014 61.11
2015 49.21
2016 72.05
2017 65.00

Batting second has become increasingly favourable as conditions have become more batsman-friendly. "When aren't they set up for batsmen? Small boundaries, flat tracks… yeah, it's just how it is nowadays," Tymal Mills lamented before the England-India T20I series.

Average scores in T20 cricket have nudged up in recent years - from 152 in the 2011-13 period to 159 in 2014-16, according to Finlay - making it harder for teams batting first to gauge what a match-winning total is.

Win percentage of chasing teams, by year (excluding ties/no-results)
Year Wins Losses Total Win %
2003 21 27 48 43.73
2004 24 24 48 50.00
2005 31 39 70 44.28
2006 41 50 91 45.05
2007 56 53 109 51.37
2008 108 85 193 55.95
2009 113 122 235 48.09
2010 152 155 307 49.51
2011 133 143 276 48.18
2012 159 141 300 53.00
2013 149 146 295 50.51
2014 176 157 333 52.85
2015 152 154 306 49.67
2016 186 154 340 54.70
2017 25 15 40 62.50

That has been Eoin Morgan's experience for Sydney Thunder this BBL, and indeed, around the world. "The level of skills batsmen have shown, the improvement dealing with levels of risk and finding boundaries means batting units or teams in general are quite comfortable knowing what their target is. The unease of posting a score, or knowing what a good score is, is becoming more and more difficult," he said. "Probably over the last three years, having a look at previous results or scores at the ground hasn't been as reliable as in the past. That's made it difficult."

So as batting has improved, precedent at a ground no longer provides a useful guide to what is a match-winning score. There is no real method, other than gut, for determining what is, say, a 175 ground and what is a 190 one. Teams batting first are at risk of undershooting, as when India made 192 for 2 in the World T20 semi-final, with Ajinkya Rahane performing his role - only, it turned out, it was a superfluous role - to perfection in making 40 from 35 balls. Increasingly they are also at risk of doing the opposite, and being so used to aiming for 200 that they fail to recognise when 150 is a match-winning score, and misjudge their plans.

Teams also now have greater batting depth. Bowlers increasingly recognise how they can make themselves more useful - and more likely to win lucrative contracts - by adding lower-order six-hitting to their games. Ben Hilfenhaus, who spent most of his professional career batting at No. 10 or 11, recently blitzed 32 not out to win a BBL match that appeared lost.

"Bowlers can't afford not to bat in modern T20. They need to be able to slog a few," says Alex Wakely, the captain of Northamptonshire, the reigning T20 champions in England. "We prefer chasing because we bat so deep." Such depth allows teams to calibrate their run chases more effectively, and the greater proficiency of lower-order players can also liberate a side's top-order batsmen to attack more at the start of a chase.

Batting second also makes it easier for a side's bowlers. "The opportunity to bowl first also means you can usually stick to your bowling plans," says Charlie Burke, director of cricket for Cricket Hong Kong. For instance, having a spin bowler bowl two early overs and then return in the 14th and 16th overs, or ensuring that the final over is entrusted to a particularly skilful bowler. But when a side is bowling second, their plans have to adapt depending on the circumstances of the match - there is no sense in leaving the final over to the best bowler if the match looks to be on its way to being lost. When bowling second, bowlers are more likely to be forced to bowl at a time that does not ideally suit them.

There are some exceptions to this new bat-first preference. In the recent Desert T20, the T20 competition for Associates, ten out of 15 teams who won the toss opted to bat first - though nine of the 15 games were still won by the chasing team. Associates are less inclined to chase because teams tend to have less depth in hitting all the way down the order, believes Burke, though that could change as emerging nations gain experience.

And when wickets occasionally revert to favouring bowlers, it becomes much trickier to chase. During last year's World T20, the ground at Nagpur notably offered more assistance to bowlers than other venues; six of the nine matches there were won by the team batting first. But on the batting-friendly pitches that are increasingly the norm, batting second is a boon. So far in this year's Big Bash League, 61.8% of matches (21 out of 34) have been won by the side batting second.

"Probably over the last three years, having a look at previous results or scores at the ground hasn't been as reliable as in the past. That's made it difficult to set a target"
Eoin Morgan, England's limited-overs captain

There are areas in which bowlers can fight back, which might yet make captains more inclined to bat first after winning the toss. Innovators like the Bangladesh left-arm fast bowler Mustafizur Rahman, who has an astounding variety of deliveries, and Yasir Jan, an ambidextrous fast bowler from Pakistan, could shift the balance of T20s a little more in favour of bowlers. And a recent tweak to the laws of the game restricting the size of bats, which will be in force from October, could also make a modest difference.

Yet it seems more likely that the preference for batting second will increase further. "Teams are so heavily packed with power all the way through a batting line-up that chasing will become the norm," Wakely believes; pitches in T20 cricket are also likely to continue to improve. Since the start of 2016, chasing teams have won 11% more T20s than those batting first, suggesting the advantage is becoming greater. Should the proportion of matches won by the side batting second continue to increase, that would make the toss ever more important in T20.

The shortest format thrives on unpredictability; too much onus on the toss, as in this year's BBL, risks stripping T20 of a little of its excitement. If the trend in favour of batting second indeed continues, perhaps cricket's authorities could even become amenable to radical suggestions - for instance, sealed bids, with captains bidding a certain number of runs for the right to bat or bowl first - to reduce the importance of the toss. In the meantime, rarely in the history of T20 has winning the toss, and the right to bat second, been as coveted as it will be when Adam Voges and Moises Henriques walk out with the coin in Perth on Saturday.

Tim Wigmore is a freelance journalist and author of Second XI: Cricket in its Outposts