According to a new study by Meredith Wills, a Society for American Baseball Research award-winning astrophysicist, the league used two distinct types of baseballs — one lighter and deader than the other — during the 2021 season. By dissecting and carefully measuring hundreds of balls used in 15 major-league parks, Wills found that the league did indeed introduce a new ball with a lighter center, as it pledged to do in the February memo. But she also found that MLB continued to use the older, heavier-center ball at the same time, apparently without telling fans, clubs, or players.

Wills' findings could have far-reaching implications for the sport, shattering what little trust and goodwill remains between league officials and players, whose fortunes rise or fall on minuscule and obscure details like the weight of a baseball's center. Informed by Insider that the league sent teams two different balls with different performance profiles, players, scouts, and front-office staffers expressed bafflement and frustration. "There's a lot of things that ... the public doesn't really see," said one National League outfielder who asked not to be named. "There's just, like, lack of transparency, with [these] issues. And, you know, I don't know why that is."

Wills, whose work to reverse-engineer the design and manufacture of baseballs has been covered in Sports Illustrated and The Athletic, has been collecting and deconstructing game balls since 2018 to track changes that the league — which purchased Rawlings in 2018 alongside a private-equity firm — makes. When Wills took apart balls used last season and weighed their centers, she found that just under half clustered at about 124 to 125 grams, roughly in line with the league's new process for balls with a lighter center. The other balls, however, had centers clustered at about 127 grams, consistent with all the baseballs she had cut open from the previous 20 seasons.