Over the last couple of years, the massive home-run spike that began in the second half of 2015 has been the biggest story in baseball. Jeff just noted the other day that home runs are once again trending up quickly, even relative to the new recent norms, and the home-run era is showing no signs of leveling off.

In trying to find an explanation for the sudden and massive increase in home runs, the ball has always seemed like the most reasonable explanation. No one has done more work on whether the ball is at the center of the home-run spike than Ben Lindbergh, who did a deep dive on the issue at FiveThirtyEight last summer, then gotaccess to some results of MLB’s internal study on the issue a month ago, putting something of a damper on the ball as the culprit.

Today, though, Ben is back with a new piece, and based on some research commissioned by Mitchel Lichtman, there again appears to be some evidence that the ball has changed the last few years.

The newer balls have higher CORs and lower circumferences and seam heights, which would be estimated to add an average of 7.1 feet to their distance, equivalent to the effect we would expect to stem from a 1.43 mph difference in exit speed. Although those differences don’t sound enormous, Nathan has noted that “a tiny change in exit speed can lead to much larger changes in the number of home runs.” Last July, he calculated that an exit-speed increase of 1.5 mph would be sufficient to explain the rise in home runs to that point, which means that the 1.43 mph effective difference that Lichtman’s analysis uncovered could comport almost exactly with the initial increase in home runs. Lichtman calculates that a COR increase of this size, in this sample, falls 2.6 standard deviations from the mean, which means that it’s extremely unlikely to have happened by chance.

Alan Nathan, the foremost expert on baseball physics out there, did offer a response on Twitter that this is still not an open-and-shut case.

The effect of a small change in seam height on fly ball distance may be overstated in the article. See https://t.co/YcLUhtvFUB, Fig. 5. 1/2 https://t.co/iooacinKUz

— Alan Nathan (@pobguy) June 14, 2017

The effect of a small change in seam height on fly ball distance may be overstated in the article. See https://t.co/YcLUhtvFUB, Fig. 5. 1/2 https://t.co/iooacinKUz

— Alan Nathan (@pobguy) June 14, 2017

But with nearly every other reasonable cause for the spike in home runs, and the speed at which things changed after 2015, it’s still difficult to reconcile current home-run levels with anything besides some change in the ball. Ben and MGL’s data provides a bit more evidence that the ball is maybe at least part of the explanation. I definitely encourage you to read their entire piece, as it’s some of the best baseball research done in the public sphere.