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Image credit: © Adam Hagy-USA TODAY Sports

For extended interviews with Committee Chair Dr. Alan Nathan and Committee Member Dr. Dan Brooks, download or stream Episode 24 of The Stolen Signs Podcast.

On July 17, 2015, Mike Trout hit one of the first home runs after the All-Star break. It was a walk-off shot to left-center field off of then-Boston Red Sox closer Koji Uehara, and it began a surge in the number of home runs that would accelerate for the next two seasons. Last September, Alex Gordon capped off that surge by knocking the 5,694th dinger in 2017, setting the all-time record for home runs in a season.

In between Trout and Gordon, observers proposed innumerable explanations for the rising tide in homers, from weather to a league-wide shift in offensive philosophy, most notably through anecdotal improvements made by hitters increasing the launch angle of their batted balls. But after months of investigation, a task force of scientists commissioned by Major League Baseball released their report on Thursday and confirmed that the ball itself is largely responsible for the increased number of home runs. While the results of the study are striking, the exact mechanism of the observed changes is yet to be explained.

The commission, chaired by baseball physicist Alan Nathan, found that air resistance was the dominant factor that changed, driving hundreds of additional home runs in the last three years. The report details the results of dozens of new analyses on data collected by Rawlings, MLB, and scientists on the commission itself.

The report confirms many aspects of research Rob Arthur and others have published in the last three years. That research examined and physically tested baseballs for differences in structure, air resistance, bounciness, and weight—factors that might contribute to the home run surge by making the ball come off the bat faster and fly further. Using data sources from PITCHf/x to x-rays, we discovered that balls manufactured in the last two years were structurally different than in previous years and significantly more lively.

In particular, the commission pins the largest part of the home run surge on decreased air resistance, or drag. Using several sources of data, including direct measurements of drag, the commission concludes that a drop in air resistance propelled batted balls on typical home run trajectories a full six feet further, more than enough to explain the huge increase in homers since 2014.


(Figure 50 of the Report of the Committee Studying Home Run Rates in Major League Baseball)

An alternative explanation for the home run surge is that players adopted a new approach to swinging that produced more hard-hit fly balls, a hypothesis called “the fly-ball revolution.” The committee’s report discards that explanation, noting that median launch angle has barely budged in the last few years. There is no doubt that some individual players have benefited by modifying their swings to produce more hard fly-ball contact, and these stories have often received national attention. Meanwhile, to less fanfare, many others have suffered as well, and the overall effect on home run rates appears to be negligible. The commission likewise notes that weather is insufficient to explain the boost in home run rates.

The report does not confirm every finding of our previous research. While Rawlings admitted to the commission that some aspects of the core production changed in 2015, the report concludes that weight likely had little or no impact on the home run surge. Research by Rob Arthur and Tim Dix found that reduced baseball weight could add as much as six inches to the flight of the ball. Similarly, the commission’s report sees the bounciness of the baseball as adding very little distance to the typical fly-ball trajectory. In both cases, our results may have been off because we could only afford to test a handful of baseballs. In contrast, MLB made hundreds of thousands of Statcast and HitF/X data points available to the commission for testing, as well as many official game baseballs.

In a press release, MLB announced a set of actions they would take to monitor and control air resistance going forward. Another crucial recommendation of the committee is that baseballs be stored in a constant temperature and humidity environment before use, a proposal MLB already appears to have acted upon by issuing new storage rules for teams to follow. Such rules will standardize the conditions of the baseballs and reduce the variation in coefficient of restitution between individual balls.

Despite the deep and meticulous analysis performed by the committee, the report does not offer firm conclusions about what aspects of the baseball changed to reduce air resistance. Many factors, from the height of the seams on the baseball to the circumference of the ball, can influence drag, and it’s not clear what combination of changes caused the drop in air resistance that boosted home runs so significantly.

Before we explore the causes, it should be noted that there is no indication of nefarious actions on the part of MLB, Rawlings or any other contributor to their supply chain, as examined by the committee. The open access provided by MLB and their partners have only shown a great deal of diligence and expertise. While the process of manufacturing the baseball has changed slightly in the last three years, there is no indication that Rawlings’ changes caused the reduced air resistance and home run surge.


(Figure 42 of the Report of the Committee Studying Home Run Rates in Major League Baseball)

The commission hints at two intriguing possibilities for why the new baseballs seem to slip through the air so easily. One is that the surface texture of the baseball has changed, an accusation leveled by multiple pitchers during the 2017 World Series. A slightly rougher or smoother surface might influence a pitcher’s ability to grip and spin the ball, potentially influencing the break on curveballs and explaining the preponderance of blisters in recent years.

The second idea is that the baseball’s center of gravity may have changed, resulting in complex aerodynamic effects that could drive batted balls further. The report’s authors note that more uniform winding of the yarn around the center of the baseball could have changed the center of mass within the ball, potentially resulting in decreased drag. It’s not clear how this kind of change might impact pitchers, but it may also fit with their description of the baseballs as sometimes “lopsided” or “flat.”

Rawlings did not measure drag during the quality control processes, nor was it something MLB requested previously. Despite a long history of play at high altitude and dry conditions, MLB did not impose standards for ball storage until recent years. A humidor and a wind tunnel purchase in 1995 may have saved us all a lot of confusion.

The committee’s transparency in releasing the report gives some closure on the many outstanding questions from the home run surge. Most importantly, it shows that MLB and Rawlings take the concerns about the ball being different seriously, and are willing to take this opportunity to implement new procedures to better control the baseball. In the end, the report may end up spurring the development of a more consistent and even playing field. The work in this matter has only just begun.

Baseball players have understood the possibilities of modifying the physical state of the ball since the origin of the sport, when some handmade balls were livelier than others. The ball has evolved countless times in the decades since, seeing regulation, cowhide, cork, rubber, and saliva. But the narrative of the baseball being “juiced” since 2015 began with the idea that MLB was tampering with the hidden center of the ball, and the amount of springiness that center provides. In light of today’s report and previous research showing that the ball became less air resistant, it may be time to retire the phrase. The modern baseball is not “juiced,” but for reasons not yet discernible, it has certainly become more aerodynamic.

Regardless of the cause, the consistency of the ball requires oversight. Baseball, more than any other sport, has a contract with its own history, and that contract depends on maintaining a transparent consistency with the balls that have been thrown and hit before. Given the frustration voiced by prominent MLB stars like Justin Verlander, their response is an important unknown factor. This report is the first time MLB has officially confirmed that the baseball may be to blame for the home run surge and the massive effects it wrought on the sport. As MLB begins to adopt new procedures to monitor and store the baseball, it’s likely that the game will enter another new era.

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