At least four separate independent tests suggest that MLB baseballs used after the 2015 All-Star Break
may be significantly enough different to explain the huge increase in HRs since then. An exhaustive report by
FiveThirtyEight.com indicates that the three basic differences are: 1.) a lighter, more compact core in the newer baseballs; 2.) tighter seams and 3.) more "bounciness". Taken together, these differences could result in balls traveling 8.6 feet further than before. Also, these differences could explain what
should have been a 25% increase in HRs—about half the actual 46% increase between 2014 and 2017. Other factors—like a changing hitting "philosophy" favoring upper-cut swings—could account for the rest of the increase. Interesting stuff. Of course, MLB continues to deny their baseballs have been "juiced".
