As Home Run Rates Rise, MLB Offers Evidence That the Ball Isn’t Juiced

Monday night in Major League Baseball looked a lot like many other nights in baseball’s highest home run era. Nine games were played, and 25 home runs were hit — an average of 1.4 per team in action, some from the usual suspects (Bryce Harper, Joey Votto, Giancarlo Stanton) and others from less likely sources (Drew Butera, Ryan Goins, Cory Spangenberg). Mark Trumbo went deep on the highest pitch to be hit for a homer since at least 2007. Ryan Schimpf, an uppercut king who might have the highest fly ball rate in history, hit another fly ball over the fence. Marcell Ozuna and Jed Lowrie touched ’em all twice each.

I noted in March that baseball’s home run rate was up slightly in spring training, relative to last spring. Through the first five-plus weeks of the regular season, the same has held true. However you slice the stats — plate appearances per home run, home runs per batted ball — last season’s home run rate was the highest ever. Thus far, this year’s has been higher still, even though home run rates are usually lower early in the season, before the weather warms up and the ball flies farther. All indications are that we’re headed for another new season-long high.

On the face of it, there’s nothing suspicious about the fact that MLB batters are hitting home runs more often than ever. Major leaguers have been getting bigger and (presumably) stronger since the sport went pro, a gradual process that seems to have reached its logical end in the giant Aaron Judge. Pitchers are throwing harder, and with data-driven teams placing less emphasis on making contact, hitters have less reason to ease up on their swings. Some hitters, at least, have used ball- and swing-tracking technology to tinker with their launch angles, aiming for more optimal, fly-ball-oriented outcomes.

What raises red flags is how rapidly the home run climb has occurred. Most large-scale changes in baseball take time. Other trends, such as the spike in strikeouts, the slowing and lengthening of games, the decline of counterproductive small ball, or even the increasing frequency of infield shifts, have happened over periods ranging from a decade to much more than a century. Home runs, though, seemed to flock back into baseball almost overnight. In 2014, scoring sank to its lowest levels in a full season since 1976, prompting speculation that MLB would intervene to bring back offense and keep fans engaged in games. Through the first half of 2015, scoring stayed suppressed, but with almost too-convenient timing, it rebounded right after the All-Star break, largely driven by a substantial rise in batted-ball exit speed and, in turn, home run rate. The rising tide lifted all hitters, even the comparatively small and weak ones.

This dramatic midseason change, which was sustained in 2016, seemed too sudden for PEDs, swing changes, more aggressive approaches at the plate, or any of the other theories that have been bandied about to explain satisfactorily, which led me and many others to conclude that the most sensible single explanation was a change to the baseball itself. (Some coaches and players, past and present, concurred.) The stats seemed consistent with a new ball being introduced at the 2015 All-Star break and phased in leaguewide as teams’ supplies of old balls were exhausted. As Baseball Prospectus writer Matthew Trueblood put it on Monday, “We can hem and haw about the lack of perfect evidence, and we certainly aren’t going to get commissioner Rob Manfred to admit it or anything, but the league juiced the baseball in the summer of 2015, and they haven’t unjuiced it.”

In a meeting last July, Manfred told a group of baseball writers (including ESPN’s Jerry Crasnick) that MLB had done “extensive testing on the baseballs and found no changes.” However, he didn’t offer any support for that statement, even though there is precedent for MLB sharing the results of its testing, as the league did via press release in June 2000, the highest-home-run-rate year before 2016. To my knowledge, no other member of the media has been offered any evidence that the ball hasn’t changed in the past two years, beyond MLB’s say-so. Rob Arthur and I commissioned our own testing of 2014 and late-2015 balls for a March 2016 article, which didn’t reveal a large difference, but the samples were too small to be conclusive.

Since early last year, I’ve made periodic requests for a copy of the results Manfred cited in July. On Monday, MLB complied with that request. As a condition of that access, I can’t release the report. However, I can relay its content and conclusion, which back up the commissioner’s comments and indicate that the baseball hasn’t been altered in any way that would explain the dramatic dinger surge.

The report is an 11-page document that summarizes the results of testing conducted in July 2016 and February 2017 by the Baseball Research Center at the University of Massachusetts Lowell, which has overseen MLB’s ball testing since the late 1990s (separate from additional testing by baseball manufacturer Rawlings). The document describes the procedures that the BRC performed during each of these periods, reports the results, and compares them to previous values generated by BRC tests that in most cases were conducted twice a year from 2004 to 2008 and three times a year ever since.

In the 2016 study, MLB received two dozen balls apiece from five teams, with each club’s contribution coming from its Rawlings-supplied stock designated for in-game use during the regular season. The BRC tested a dozen balls from each shipment immediately and stored the other dozen under controlled atmospheric conditions for at least two weeks before beginning the tests, which consisted of measuring the balls’ weight, circumference, and coefficient of restitution, or COR, a measure of elasticity (basically, bounciness) that largely determines how fast and how far a batted ball travels.

The report says that all of the baseballs were not only well within the limits of MLB’s fairly broad manufacturing specifications, but also “comparable in weight, circumference and COR to previous compliance data,” an assertion supported by plots of average values and 95 percent confidence intervals for each of the previous testing periods. The COR graph below — which I requested (and was granted) permission to share — is one of the document’s many nonsmoking guns. (The squiggly blue line marks a point in the 2012 offseason when the test conditions switched from balls being fired at 85 feet per second into a solid ash block to balls being fired at 88 feet per second into a solid steel plate.)

Based on the BRC’s findings, the report concludes, “There is no evidence from the results of this study that the performance of the 2016 regular-season baseballs used by these five clubs would have resulted in any difference in on-field performance from those used during recent seasons.”

Late last summer, MLB asked Alan Nathan, a professor emeritus of physics at the University of Illinois and an expert on the physics of baseball, to review the BRC’s research and offer, in his words, an “honest and independent opinion.” (Disclosure: I’ve edited articles by Nathan, solicited his input for articles of my own — including articles about the recent rise in home run rate — and had him on podcasts.)

“Quite frankly, I was disappointed at that result, because I was hoping I’d find something,” Nathan, who was compensated by MLB for the time he spent studying the BRC report, tells me by phone. However, he says, “I saw nothing in the data that was presented that suggests that the ball has been altered at all.”

The report’s February 2017 results are based on a sample of eight dozen baseballs sent to the BRC by Rawlings. This more recent report includes weight, circumference, and COR results in addition to data on ball hardness, seam height — another factor that can affect the flight of the ball — and seam-height variance, and CCOR (cylindrical COR) testing, which measures elasticity in collisions at higher speeds against a non-flat surface to come closer to mimicking in-game conditions. Again, the BRC concluded that the balls weren’t out of line with MLB specs or previous values.

“It’s a puzzle,” Nathan says about baseball’s home run explosion. “I can’t explain it. … I’m not more [mystified] than I was before, but I’m still mystified.” In background conversations with me, baseball officials haven’t claimed to have definitive answers either.

Up until Tuesday, we’ve had to take the league’s word for it that there was a study that ostensibly absolved the ball of blame. Now that we know the study is real, we have to take the league’s word for it that the tests were conducted as described and that the data is accurate and complete. Nothing in the apparently thorough report raises doubts, but it can’t hurt to remember that it would be in MLB’s best interests to establish that the ball hasn’t changed.

Any artifice would be difficult to detect from afar, because baseball testing is expensive, and outside sources don’t enjoy MLB’s access to game-ready balls. But falsifying or skewing — and voluntarily producing — a detailed report instead of just staying silent sounds like a lot of work for little return. (Cue the conspiracy theorists: “That’s what they want you to think.”) This April’s attendance was up compared to 2016, and franchise values are still skyrocketing. Baseball’s finances are flourishing. Yes, it would have looked somewhat strange if MLB had kept declining to release the report while claiming to have nothing to hide, but only to people who were paying close attention. Many fans may not know or mind that home runs have become much more common, and the questions that some writers have raised about MLB putting its thumb on the scoring scale haven’t hurt the sport in any obvious way. Occasional calls and emails from me aren’t exactly a congressional inquiry, and the MLB spokesman who sent the report said that he hadn’t received any other requests. It doesn’t seem as if MLB has much incentive to tell the kind of lie that could come back to bite a commissioner.

If the sport’s home-run-happy present has nothing to do with the ball, then the commissioner has some implications to consider. MLB’s seasonal strikeout rate, which is on track for a 12th-consecutive increase, has climbed by more than a percentage point since the low-offense season of 2014, and non-homer runs are increasingly rare. If not for the huge hike in home runs, we could be looking at line scores straight out of the deadball era by now. And if that hike hasn’t come from the baseball, it might not last as long.

Pitchers can’t de-juice a baseball, but they can counter a change in batters’ approach. If the spread of dingers has less to do with COR or seam height than with a wave of Yonder Alonso–like breakouts by hitters who’ve tailored their swings to lift low pitches, then pitchers could exploit those uppercuts by raising their own sights and firing fastballs up in the zone, especially if the bottom of the strike zone is finally contracting and low strikes get more scarce. According to data from Pitch Info, this season’s average four-seamer height so far is the highest since 2011.

The historic performance we’ve seen since mid-2015 still supports at least a little skepticism about the true roots of baseball’s home run revolution; without witnessing the tests, we can’t consider these findings definitive. But the “juiced ball” hypothesis does seem much less likely than I thought it did two days ago. “It has every look of being suspicious,” Nathan says about the timing of baseball’s big-fly bailout. “But as I said, there’s nothing I could find that suggests anything amiss.”