From an MLB press release:

Major League Baseball has received the results of a study conducted by a committee of scientists on the causes of the increased home run rate in the game since 2015.  The committee, assembled by Commissioner Robert D. Manfred, Jr. in August 2017, was comprised of the following experts:

• Alan Nathan (Chairman) – Professor of Physics Emeritus, University of Illinois
• Jim Albert – Professor of Statistics, Bowling Green State University
• Jay Bartroff – Professor of Mathematics, University of Southern California
• Roger Blandford – Professor of Physics, Stanford University
• Dan Brooks – Owner of BrooksBaseball.net
• Josh Derenski –Ph.D Student, Marshall School of Business, University of Southern California
• Larry Goldstein – Professor of Mathematics, University of Southern California
• Peko Hosoi – Professor of Mechanical Engineering, Massachusetts Institute of Technology
• Gary Lorden – Professor Emeritus of Mathematics, California Institute of Technology
• Lloyd Smith – Professor of Mechanical & Materials Engineering, Washington State University

The committee investigated a number of hypotheses, including the properties of the baseball, weather conditions and changes in player behavior.  The research included a detailed analysis of Statcast data, as well as an investigation of the properties of the baseball that can affect home run production, including an inspection of Rawlings’ production plant in Costa Rica.  The committee set its own agenda and arrived at their own conclusions and recommendations, independent of the Commissioner’s Office.

The committee concluded that the increase in home run hitting since the 2015 season was due, at least in part, to a change in the aerodynamic properties of the baseball (i.e., reduced drag for given launch conditions, as opposed to a change in launch conditions).  That conclusion is supported by their analysis of Statcast data, a physics-based model that the Committee developed, and laboratory testing of game-used baseballs from before and after the increase in home run rate.  The Committee did not find any change in the size, weight, seam height, or COR of the baseball that would explain the increase in home runs.  Though the Committee was unable to conclusively prove the exact cause of the reduced drag since the 2015 season, they offered hypotheses including that the rubber pill may be more centered within the baseball since 2015 and that the ball may be staying rounder while spinning since the 2015 season.

Importantly, the committee also concluded that no change to the materials or manufacturing process, whether intentional or unintentional, has played a significant role in the home run surge.

Based on the results of the study, the Commissioner is taking the following actions:

 

1. Monitoring of Temperature and Humidity Conditions. The Commissioner’s Office is monitoring temperature and humidity in the ball storage locations of the 30 ballparks and will work with the committee on whether to require the use of humidors at all ballparks for the 2019 season.
2. Review of Production Specifications of Baseballs.  The Commissioner’s Office is working with Rawlings to make updates to the existing production specifications of baseballs and to develop additional specifications for the aerodynamic properties of the ball.
3. Perform Aerodynamic Testing on Baseballs.  In addition to the existing testing protocol, the Commissioner’s Office is working with the committee to develop a set of aerodynamic tests for game baseballs.
4. Create Standards for Mud Rubbing.  The Commissioner’s Office is providing Clubs with guidance on the appropriate mud rubbing of baseballs, which will be enforced by the umpires.
5. Formation of a Scientific Advisory Council.  Some members of the committee will continue to advise the Commissioner’s Office on issues related to baseball performance, as well as other science-related topics.

Commissioner Manfred said: “I thank the committee for all of its hard work on this important issue.  Based on the results of their study, I am accepting their recommendations immediately and look forward to their continued guidance in this area.”

The full report is here.