Ten years of Japanese baseball data settle the designated hitter debate - sort of

Nagoya University

Does adding a tenth batter to a baseball lineup change who wins? The designated hitter rule, which lets a specialist hitter bat in place of the pitcher, has divided baseball opinion since the American League adopted it in 1973. Purists argue it dumbs down the game by removing strategic decisions about when to pull a pitcher for a pinch hitter. Proponents say it produces better offense and protects pitchers from injuries. Both sides have had plenty of opinions. Neither has had much data.

A new study from Nagoya University, published in PLOS One, provides some. The answer, drawn from 10 years of Japanese professional baseball, is both clear and anticlimactic: the DH rule does not change whether talented teams win games.

Japan's Pacific League as a natural experiment

Japan's professional baseball structure offered the researchers something unusual: a controlled comparison. The Pacific League has used the designated hitter since 1975, while the Central League has not (though it plans to adopt the rule in 2027). When Pacific League teams play home games, they use the DH. When they play interleague road games in Central League stadiums, they do not.

This means the same teams play under both rule sets within the same season - a natural experiment that separates the effect of the rule from the effect of the league. Researchers Shino Shimizu and Associate Professor Yasuhiro Suzuki, from Nagoya University's Graduate School of Informatics, analyzed game data from all six Pacific League teams across the 2014-2023 seasons.

Their statistical analysis found no significant difference in how player performance translated into wins under the two systems. Teams with better players won games at essentially the same rate regardless of whether they had a designated hitter in the lineup. The DH changes the composition of the batting order. It does not change the outcome equation.

A new way to measure what players are worth

The study's methodological contribution may outlast its headline finding. The researchers developed an improved version of WAR - Wins Above Replacement - the statistic that estimates how many wins a player contributes compared to a freely available substitute.

Traditional WAR calculations use the same mathematical baseline for all positions. A replacement-level catcher is treated as equivalent to a replacement-level first baseman. But in reality, performance varies enormously by position. Catchers, on average, are weaker hitters than first basemen. Shortstops are weaker hitters than corner outfielders. A player who bats .260 at catcher is far more valuable than one who bats .260 at first base, because the available replacements at catcher hit much worse.

Shimizu and Suzuki addressed this by measuring both starting and backup player performance at each position using actual game data from the Pacific League. They also tracked designated hitters as their own category, since DH players bat but never field. This position-specific approach produces a more accurate picture of each player's contribution than the traditional one-size-fits-all baseline.

What the Pacific League's reputation gets wrong

The Pacific League has long been considered the stronger of Japan's two professional leagues. Some analysts and fans have credited the DH rule for this, arguing that the extra batting position produces better offense, more exciting games, and ultimately superior team development.

The data do not support that explanation. If the DH rule were genuinely making Pacific League teams better, you would expect to see a measurable difference in how player talent converts to wins between DH and non-DH games. The study found no such difference. Whatever explains the Pacific League's competitive edge - and it may simply be organizational and scouting differences between the leagues - the DH rule does not appear to be the mechanism.

This finding speaks to a broader pattern in sports analytics: rule changes that feel significant to fans and commentators often have smaller competitive effects than assumed. The DH unquestionably changes the experience of watching a game. It changes lineup construction, bullpen management, and late-inning strategy. But it does not change the fundamental truth that teams with better players tend to win.

Six teams, one league, and the limits of the data

The study has constraints that affect how far its conclusions can travel. The Pacific League has only six teams, and the dataset covers only their games over 10 seasons. While this provides a substantial number of individual games, the team-level sample size is small by statistical standards. Effects that are real but modest might not reach statistical significance with only six teams as data points.

The analysis also focuses exclusively on the Pacific League. The researchers note that their method could be applied to Central League teams to predict the effects of DH adoption in 2027, but they have not done so in this study. Whether the same null finding would hold in Major League Baseball - where the universal DH was adopted in 2022 across a 30-team league with different talent levels, salary structures, and competitive dynamics - is an open question.

Japanese and American baseball also differ in ways that could affect the DH's impact. Japanese leagues have different roster sizes, salary constraints, and player development systems. The role of the pitcher as a batter may carry different strategic weight in a league where teams play 143 games per season versus 162. These contextual factors limit direct comparisons.

Finally, the study measures competitive outcomes - wins and losses - but does not address other arguments for or against the DH. Fan engagement, game pace, pitcher injury rates, and the aesthetic quality of play are all factors in the DH debate that this analysis does not touch.

What the Central League should expect in 2027

For Japan's Central League, which plans to adopt the DH rule in 2027, the study offers a specific and somewhat reassuring prediction: the change will alter how teams construct their rosters without fundamentally disrupting competitive balance. Teams that are good will remain good. Teams that are bad will not be rescued by adding a designated hitter.

First author Shimizu, a baseball fan and statistics researcher, puts it concisely: the DH rule changes the game experience, but it does not alter baseball's fundamental competitive balance. Teams with better players still win.

That conclusion will satisfy neither side of the DH debate. Purists will note that the study does not address their core complaint - that the DH removes an interesting strategic dimension from the game. Proponents will argue that better offense and pitcher safety are worth pursuing even if competitive balance is unchanged. The arguments will continue. They always do in baseball. But at least now there is data in the room.