Beginning in 2015, Major League Baseball has Witnessed More Home Run than Ever Before

The analysis of year-to-year home run (HR) ratios was first compiled to understand how the current trend compares within the last 26 years ( Figure 1 , left panel). Season statistics were obtained from baseball-reference.com mlb.com , and foxsports.com . To account for the differences in gameplay over the years, HR per game was calculated since the beginning of the PED era (1994–2004) to 2019. The 2015 season registered a noticeable trend that started in the MLB when HRs began increasing dramatically and continued through the 2019 season. This is contradictory to the regression observed starting from the 2004 season and ending in 2014, when 2014 registered a 26 year minimum. What makes this trend more interesting is that the 2016 season was just shy of breaking the all-time major league record for cumulative single-season HRs, whereas in 2017, the reigning league HR record, achieved in 2000, was surpassed by 9%. This is a stark difference compared to the aforementioned 26 year minimum observed just a few years back in 2014, resulting in overall a whopping 46.5% difference in the number of HRs hit in the short time frame of the 2014 and 2017 seasons.

Figure 1 Figure 1. Among all of the US professional baseball leagues, only in MLB did the seasonal league home records exceeded yearly average. MLB home runs exceeded yearly averages since the 2016 season. Annual home run records were collected for professional baseball leagues (top left) MLB (top right) and Minor League Divisions from 1994 to 2019. The PED era is referenced from 1994 to 2003 where PED testing started in 2003. The 2019 AAA season was circled in green to indicate the initial use of the MLB baseball compared to the minor league baseball that has been used the 25 seasons before where a 52% increase in home runs has occurred. (Bottom) A tabulated version of A and B showing the 20 year average; post-PED era average (2004–2015); the 2016–2019 seasons home run per game totals.

Professional baseball in the United States has a number of professional development leagues that serve as a farm system for MLB. The company Rawlings produces all of the baseballs that are used in the Major and Minor League systems, but the Major League uses balls entirely from the manufacturing unit in Costa Rica, whereas the Minors use balls produced in China. (7) When the year-to-year averages across the different leagues are examined, it became rather clear that the unusual surge in HRs was observed only in the MLB may be linked to the baseballs. Extrapolating HR per game across the last 26 years in the Minors, as in Figure 1 with the Majors, we see similar trends up to 2014, but clearly, the recent surge in HRs exceeding historical records was not observed in the analysis (except in 2019 for the AAA). All Minor league divisions registered peaks of HR per game during the PED era and regressions from 2004 to 2015 (specifically a 3% reduction in AAA and AA, which was identical to the MLB) following the trend in MLB, highlighting the consistency between the four professional leagues. All three minor league divisions also showed an upward trend of HRs in recent years, but the HR/game ratios were are all equal or less than the 20 year average and similar to the 2004–2015 average, as shown in Figure 1 (bottom). Interestingly, as of the 2019 season, the AAA Minor League started using the MLB-issued baseball. As of May 16th of 2019, AAA players were hitting 1.34 HR/Game, which was a 52% increase from the previous year. This stark difference with the addition of the MLB baseball and the lack of difference in similar HRs trends in the other minor league systems only further emphasized the tremendous increase in HRs observed in the MLB and indicated the first possible sign correlating this occurrence to the baseballs. Other statistical factors were assessed to ensure that the increase in HRs was not the product of a gameplay factor. Figure S1 plots the average hits per game in the MLB across the same time span, showing that the last 8 years have seen a 5–7% decrease in hits per game compared to the 18 years. This is also consistent with an increase in strikeout rates, which indicate that pitching improved as time went on, possibly making hitting HRs more difficult ( Figure S2 ). When hits per HR was normalized, the same trend was observed in Figure 1 , indicating an enormous increase in the frequency of HRs ( Figure S3 ). The number of at-bats (or offensive attempts of a baseball player) per HR was also assessed to account for a small season to season deviations of at-bats due to extra-inning games or an increase in hits. The same trend existed in at-bats per HR as with the frequency of overall HR. The number of HRs hit increased in frequency, from on average every 37.5 at-bats to below every 30 at-bats starting in 2017 ( Figure S4 ).

One of the most common explanations used for the increase in HRs is a difference in the approach of batters attributed to analytics deprioritizing strikeouts. Hitters are thought to be intentionally trying to lift or raise the trajectory of the baseball more and hit more fly balls, in turn, resulting in more HRs. To investigate this theory, the relative ratio of HR per fly ball hit was again plotted over the last 26 years ( Figure 2 , left). From 1994 to 2014, the HR to fly ball ratio almost remained the same with a slight increase during the PED era, as would be expected. But contradictory to the 0.106 average ratio, during the 2017–2019 seasons, there were 47, 68, and 93% increase in the chance of a fly ball to be hit as a HR compared to the 20 years prior to the 2014 season. This is noteworthy as the variation between the highest and lowest statistical points was only 9% compared to the average from 1994 to 2014.

Figure 2 Figure 2. Despite fewer fly balls hit more home runs were being scored during the last 26 years. (Left) Year-to-year ratios of home runs to fly balls were plotted from 1994 to 2019. (Right) MLB batters groundball (GB) to fly ball (FB) ratio from 2002 to 2019 are plotted. The 18 year average ratio is 1.24 GB/FB (GB and FB stats acquired by FanGraphs, which started recording team GB and FB stats in 2002).