Houston Astros pitcher Justin Verlander has been outspoken about noticing changes in the ball itself. (Ross D. Franklin/AP)

Major League Baseball on Thursday confirmed for the first time something pitchers in the sport have been saying for more than two years: that changes to the composition and/or behavior of the baseball are responsible, at least in part, for the surge in home runs since the middle of the 2015 season. What remains unexplained, however, is what exactly those changes were, or why they occurred.

A committee of 10 scientists and data specialists, formed by Commissioner Rob Manfred, concluded in a report released Thursday that home run surge can be explained, at least partially, by “a change in the aerodynamic properties” of the ball — specifically, “reduced drag for given launch conditions.”

The home run surge “is not due to either a livelier, ‘juiced’ ball, or any change in batter or pitcher behavior,” the report concluded. “It seems, instead, to have arisen from a decrease in the ball’s drag properties, which cause it to carry further than previously, given the same set of initial conditions — exit velocity, launch and spray angle, and spin. So there is indirect evidence that the ball has changed, but we don’t yet know how.”

“The great mystery is: What in the world has happened that we’ve had a small change in drag — it isn’t large — but one that seems to be systematic enough that it’s affecting offense,” said committee member Lloyd Smith, a professor of mechanical and materials engineering at Washington State University. “We’ve done every test you could imagine, and we just couldn’t nail it down. It’s in there. It’s in the data someplace. But it’s going to take a lot more time and effort [to solve].

“Just to figure out the ‘what’ — is drag there or not? — we had to measure the drag of 22 dozen baseballs. And this was by itself a solid month of two technicians in a lab firing baseballs through the air and seeing how their drag changes.”

Though the committee was unable to prove the exact cause of the reduced drag — the “why” — it offered several hypotheses, including that the rubber “pill” at the ball’s core may be more centered in recent years, or that the ball may be staying rounder.

According to Baseball-Reference.com, MLB’s home run rate grew in every season from 2014 (0.86 per team-game) to 2017 (1.26), before falling slightly this season (1.13 through Wednesday).

The committee’s findings supported Manfred’s contention, made repeatedly the past few years, that nothing material had changed in the manufacturing or the specifications of the baseball. The committee said it found no changes to the size, weight, seam height or coefficient of restitution (or “bounciness”) that would explain the increase.

According to Smith, natural variations in the materials used to produce baseballs, including wool yarn and leather covers stitched by hand, give each ball a higher degree of variance in properties than would be found in, say, golf balls.

“While it cannot be ruled out that small year-to-year variations in these properties might be a minor contributing factor to the home run surge said,” the report said, “these changes are within normal and expected manufacturing variation.”

Analytics websites began noticing the uptick around midseason in 2015, and some pitchers, most prominently Houston Astros ace Justin Verlander, have been outspoken about what they saw as a clear change in the feel, look and/or behavior of the ball.

“Major League Baseball wants to put on a show,” Astros left-hander Dallas Keuchel said during last fall’s World Series, which saw a record 25 homers hit in a seven-game series. “Honestly, I think the balls are juiced.”

Manfred commissioned the study in August 2017, bringing together physicists, mathematicians, engineers and statisticians, who examined Statcast data, tested and compared the properties of game-used balls from 2012-17, and inspected a Rawlings manufacturing site in Costa Rica where the balls are made. They also considered “changes in player behavior” — including the trend toward hitters swinging with an increased launch angle — as a potential factor, but ruled that out.

Before this season, MLB mandated for the first time that all teams store their baseballs in air-conditioned rooms, and in a statement Thursday, Manfred said the league would study whether it needed to take the additional step of requiring the use of humidors at all parks beginning in 2019.

“We know what the ‘what’ is,” Smith said. “The ‘why’ is a question I certainly want to get an answer to. But science doesn’t always work on a time frame that’s compatible with the popular interest.”

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