Calculated using data from Fangraphs. AB is at-bats and HR is home runs.

As a big Major League Baseball (MLB) fan, I have noticed that home runs seem to be occurring with more regularity than the past this season, as exemplified by 43 year old Mets pitcher Bartolo Colon hitting his first career homer this May. The data matches my observations: during the current season, home runs are being hit more often than any other time in the history of the game, with the fewest at-bats between each home run, as shown in the chart to the right. Additionally balls are coming off of the bat faster than the past, as indicated by a greater exit velocity. I was curious about what could be driving this change and wanted to take a further look.

Evidence seems to be pointing towards a change in the ball being the primary reason for the huge increase in home runs, but there are likely a number of other factors involved. After the 2014 season, when scoring was very low and pitching dominated, the league reportedly met with the MLB Player’s Association about possible changes to increase offense, with one proposal involving a change to the ball. Last July, there began to be a dramatic increase in home runs and scoring in the major leagues. Several players have noted that the balls used in the major leagues this year seem to be wound tighter than the past, which could make it go further. It would make sense that the league would be interested in promoting more offense, since it would increase the popularity of the game. It would be strange, however, if they changed the ball without making it known to the public.

The balls used in the major leagues are manufactured by Rawlings in Costa Rica. It is possible that a slight change in the manufacturing process, which is all done by hand (with 200 workers recently laid off), could lead to increases in the coefficient of restitution (COR), a fancy physics term describing how bouncy the ball is, which consequently can produce major increases in the distance the ball travels and the speed it comes off the bat. MLB permits a wide range of acceptable restitution coefficients, which means that it is possible for balls to fall within the acceptable range, but be bouncier, and therefore, travel nearly 50 feet further.

Yoenis Céspedes of the New York Mets. Photo by Jim McIsaac/Getty Images.

FiveThirtyEight conducted an experiment that attempted to determine whether the ball has changed in recent seasons. They had official MLB balls from 2014 and late 2015 thrown from a cannon at a steel plate, in order to measure their COR or bounciness. The tests were inconclusive and did not not find a significant difference in the COR of balls from 2014 compared to those from late last season. No matter how many tests are run in a lab, there is no way to replicate game situations and ensure that the balls are exactly the same as those used in games.

Comparing the major league and minor leagues can help provide some insight into what role the ball has played in driving up the home run rate. The balls used in the major leagues and minor leagues are produced thousands of miles apart, in Costa Rica and China, respectively. Typically, the home run rates for both leagues were correlated, but recently, while homers have increased in the majors, they have decreased in the minors (Triple-A), as shown in the chart below. When analyzing matchups that took place between the same pitchers and hitters, at both the minor league (Triple-A) and major league level since last July, the the ball traveled further in MLB. 3.3 percent of balls hit were homers in Triple-A, compared to 4.3 percent in MLB. This could be a sign that the ball changed in MLB. There is precedent for this. When modifications were made to the ball in the past, there were significant changes in home run rates.

Steroids are not a logical explanation for the dramatic increase in home runs because it is unlikely that a large number of batters began taking some performance-enhancing drug at the same time that allowed them to hit more home runs, without getting caught. Plus, the league has cracked down on steroids in recent years, with harsher penalties that could result in a lifetime ban from the game.

Other theories on why home runs have increased include that a different approach is being taken by both pitchers and batters. Pitcher may be encouraged to pitch to contact to get quicker outs, while batters are being more aggressive, as shown by the decrease in walks and increase in strikeouts.  Hitters have also gotten better at hitting high velocity pitches and turning them into home runs. Research has shown that how hard a ball is thrown does not correlate with how fast it is hit off the bat, meaning that the batter is more responsible than the pitcher for the exit velocity and distance the ball travels. Therefore, the influx of young power hitters in the game could play a role in the rise in homers. Additionally, teams may be valuing power more than the past. With pitching dominating, teams may have figured that if there are less opportunities to score, they should make sure they capitalize on the chances they are given.

As we discussed in class, there are certain phenomena where it is very difficult to explain the cause, and this is a great example of that. There are no clear-cut answers for why home runs are being hit more often, but there are number of possible explanations involving variables discussed above, such as the ball and players themselves, or it could potentially be just a fluke.

Sources

• http://fivethirtyeight.com/features/are-juiced-balls-the-new-steroids/
• http://fivethirtyeight.com/features/steroids-probably-arent-causing-baseballs-power-surge/
• http://fivethirtyeight.com/features/a-baseball-mystery-the-home-run-is-back-and-no-one-knows-why/
• http://www.espn.com/mlb/story/_/id/17109063/the-mlb-home-run-explosion-2016-juiced-baseballs-blame