The Southpaw Advantage
Left-handed pitching has long been one of the most prized commodities in professional baseball. Teams strive to obtain lefty pitchers, and those pitchers recognize their competitive edge. Two-sport athlete Tom Glavine explained his career choice this way: “I love both sports, but the deciding factor was, being a left-handed pitcher, I had a huge advantage in baseball because of that, and I didn’t have that type of advantage in hockey.” Even a century ago, Tris Speaker expressed the sport’s reverence for southpaws – if falling short as a trade analyst – when he opined that “taking the best left-handed pitcher in baseball and converting him into a right fielder is one of the dumbest things I ever heard.”
The traditional explanation has pinned responsibility on baseball’s platoon advantage, in which hitters perform better in opposite-hand matchups while pitchers have the edge in same-handed contests. Since left-handed hitters have the platoon edge much more often than right-handed hitters, they have an edge in the competition for MLB jobs and end up substantially overrepresented on offense. In response, the story goes, teams then stock up on left-handed pitchers to counter the advantage offenses gain from their excess of left-handed hitters.
James Click, writing for Baseball Prospectus in 2004, noted:
“It has been true throughout baseball history that left-handed people have a spectacularly better chance than the rest of us of reaching the major leagues…. Left-handed batters (who aren’t necessarily left-handed people) enjoy a positive bias because they are more rare and because they typically have a positive platoon split against right-handed pitchers, a group that forms the majority of hurlers. Likewise, southpaws are prized because they can reverse that advantage.”
Meanwhile, C.Y. Cyrus Chu, Ted Chang, and John Chu, in their work, Opposite Hand Advantage and the Overrepresentation of Left-handed Players in Major League Baseball theorized:
“With the OH [opposite hand] advantage being in effect, there should be more than 10% left-handed batters in the league… When the number of left-handed batters goes up, the number of left-handed pitchers will also go up in order to counter the OH advantage from the left-handed batters.”
The first half of this story is generally correct. When a batter hits with the platoon advantage, his OPS is more than 80 points higher than when he faces a same-handed pitcher. And because most pitchers are right-handed, left-handed batters enjoy the platoon advantage far more often (73% of plate appearances) than right-handed hitters do (29%). The result is that over 40% of major league plate appearances in a typical year are taken from the left side, and 49% of the players at the positions not limited to righty throwers (first base, DH, and outfield) bat left-handed:
| LH Hitter | RH Hitter | LHP | RHP | |
|---|---|---|---|---|
| Platoon advantage | 73% | 29% | 29% | 53% |
| Platoon disadvantage | 27% | 71% | 71% | 47% |
The pitching component of this narrative, however, does not withstand scrutiny. It’s true that pitchers face left-handed hitters more frequently than they would in the general population, but that isn’t the relevant factor in the competition for pitching jobs. On the field, what matters is whether lefty pitchers enjoy the platoon advantage more often or less often than the righty pitchers who also seek a roster spot. The data are clear: left-handed pitchers only enjoy the platoon advantage 29% of the time, far less than the 53% rate for right-handed pitchers. Southpaws actually pay a severe “platoon penalty,” and would allow fewer runs – about 0.20 runs per game, by our estimate – if they held the platoon advantage as often as right-handed pitchers do.
Far from helping to explain the southpaw surplus, the platoon effect must actually suppress the number of left-handed pitchers. Without the lefty platoon penalty, we would expect even more pitchers to hurl from the left side. So the question remains: why are there so many left-handed pitchers in major league baseball? They should be nearly extinct, but in fact they thrive. What’s going on?
We believe that left-handed pitchers have a hidden advantage that has nothing to do with their ability to throw a baseball, but rather is solely the result of the fact that they throw with their left hand. This “southpaw advantage” is substantial enough to generate a large surplus of lefty pitchers on rosters and shape the game in profound ways. Indeed, our analysis suggests that a substantial majority of major league left-handed pitchers could not survive in the majors if they threw right-handed but had otherwise identical talent.
If you are skeptical, well, so were we. But as the kids say, we have receipts.
Southpaws Are Inferior (Except at Getting Batters Out)
At the heart of the mystery of handedness and pitching lies a crucial but under-appreciated distinction: outcomes vs. pitch quality. Major league left handers are just as successful as right handers at retiring batters, but they are not truly peers when it comes to throwing a baseball. In terms of the quality of the pitches they make – as measured by observable factors such as velocity and movement – southpaws are simply not in the same league as righties.
For this section, we used data from Pitch Info (published by FanGraphs) to look at all players who pitched at least 100 innings from 2007 through 2019, a total of 300 left-handed pitchers (143,168 IP) and 839 right-handed pitchers (379,347 IP). As you can see from the table below, the lefties and righties had virtually equal outcomes. Not only do they surrender the same number of runs per nine innings (earned and unearned) — 4.34 and 4.37, respectively — but they arrive at this result via the same path, posting virtually identical strikeout, walk and home run rates:
| Handedness | Runs/9 | K/9 | BB/9 | HR/9 | BABIP | FIP |
|---|---|---|---|---|---|---|
| LHP | 4.34 | 7.7 | 3.2 | 1.0 | .296 | 4.07 |
| RHP | 4.37 | 7.6 | 3.1 | 1.1 | .293 | 4.09 |
However, pitch quality metrics — which remove the pitcher-hitter interaction — paint a very different picture. Velocity is the single most important pitcher skill, and southpaws consistently throw slower pitches than right-handers. From 2007 to 2019, left-handers registered a substantially lower velocity for every type of pitch tracked, usually by a margin of 1.5 mph or more (see table). This includes not only higher-velocity pitches like fastballs and sinkers, but also sliders, changeups and curves. Southpaws were much less likely than right-handers to average 93 mph or more on their fastball (27% vs. 54%), and the ratio is even more extreme at the 94 mph threshold (14% vs. 38%):
| Handedness | Four-seam | Sinker | Cutter | Slider | Change | Curve |
|---|---|---|---|---|---|---|
| LHP | 91.8 | 90.7 | 86.6 | 82.8 | 82.5 | 76.7 |
| RHP | 93.3 | 92.0 | 88.7 | 84.7 | 84.2 | 78.6 |
| Difference | -1.5 | -1.3 | -2.1 | -1.9 | -1.7 | -1.9 |
Wherever we look, pitches leaving a left hand take considerably longer to reach the plate than their right-handed brethren. Still, it is widely believed that “crafty” lefties have other skills not captured by a radar gun. Perhaps southpaws offset their speed handicap with superior movement on their pitches? To answer that question, we turned to Statcast data.
As a matter of physics, rotation is required to produce horizontal or physical movement on pitches. Therefore, if southpaws are achieving superior movement on pitches, that should show up in the rate of spin on their pitches. However, we find that lefties generally have lower spin rates on their pitches, both fastballs (four-seam and sinkers) and off-speed pitches (see table). The only exception is changeups, on which lefties display a higher spin rate:
| Pitch Type | LHP | RHP | LHP Deficit |
|---|---|---|---|
| Four-seam | 2241 | 2281 | -40 |
| Sinker | 2124 | 2157 | -33 |
| Cutter | 2236 | 2400 | -164 |
| Slider | 2343 | 2421 | -78 |
| Changeup | 1836 | 1758 | 78 |
| Curve | 2420 | 2537 | -117 |
While spin rate is a critical skill, we know that pitchers can differ in their ability to convert spin into movement that helps get batter out. Theoretically, if lefties harness their spin rate more effectively than righties, they could achieve better results from the same velocity. There is no indication, however, that southpaws produce superior movement, even partially, to offset their velocity deficit. We find that five types of break are strongly associated with pitcher success: fastball vertical break, sinker vertical break, slider horizontal break, changeup vertical break and curveball horizontal break. Right-handers achieve more break on every one of these key dimensions, even after controlling for their higher velocity. Pitch movement, it appears, is just another arena of right-handed pitcher superiority:
| Pitch Type | LHP | RHP | LHP Deficit |
|---|---|---|---|
| Four-seam vertical break | -0.3 | 0 | -0.3 |
| Sinker vertical break | 0 | 0.2 | -0.2 |
| Slider horizontal break | -0.3 | 0.2 | -0.5 |
| Changeup vertical break | -0.1 | 0.3 | -0.4 |
| Curve horizontal break | -0.6 | 0.4 | -0.9 |
The performance metrics all point to the same conclusion: when a left-hander is on the mound, the quality of pitches suffers considerably. Yet this large skills gap has remained somewhat obscured and underappreciated. Why? For one simple reason: when hitters re-enter the equation, the outcomes for pitchers of either handedness are identical. At the end of the day, southpaws get the job done, so no one has cared much that they are working with less powerful tools.
This skills gap makes perfect sense given lefties’ overrepresentation relative to their share of the general population. Professional baseball dips much deeper into the left-handed talent pool, so logically it should be enlisting players with lesser throwing ability. However, the parity of outcomes must mean that some hidden factor gives southpaws a substantial advantage over right-handed pitchers that offsets this skills gap.
Southpaws’ Hidden Advantage: The Unfamiliarity Bonus
We believe the source of southpaws’ “extra” success against hitters – beyond what the quality of their pitches can explain – is hitters’ relative lack of familiarity with the look of pitches coming from the left side. As young hitters first learn their craft, they face lefties far less often than righties. This lack of familiarity reduces hitters’ ability to discern and react quickly and effectively when incoming pitches come from the south side.
Paradoxically, it is the very scarcity of lefties that creates the surplus. Or as Hall of Fame left-handed hitter Yogi Berra would perhaps have expressed it, “I wouldn’t have to hit against lefties so much if there were more of them.”
A review of the lefty advantage in other sports – both where it manifests, and where it doesn’t – generally supports this familiarity explanation. A 2000 study of university students found that those involved in competitive sports were considerably more likely to be left-handed than non-sporting students. Significantly, though, this surplus of lefties was not found in all sports:
“The excess of left-handers among sporting competitors applies only to competitors in interactive or confrontational sports, i.e., basketball, boxing, fencing, football, handball, judo, karate, table tennis, tennis, and volleyball, and not to competitors in noninteractive or nonconfrontational sports, i.e., cycle racing, discus throwing, diving, gymnastics, rifle shooting, rowing, running events, skiing, swimming, and weightlifting, in which left-handers occur about as frequently as they do in the nonsporting population.”
Other researchers have confirmed this general pattern, including at higher levels of competition: lefties are often overrepresented in interactive sports that require athletes to anticipate and respond to the actions of a competitor, such as cricket, table tennis, and fencing. In contrast, lefties consistently enjoy no advantage at all in solitary sports lacking such interaction, which also confirms that they do not possess some inherent biological advantage in eye-hand coordination, strength, or other athletic skill.
Scientists believe this pattern reflects a “negative perceptual frequency effect,” meaning that because athletes confront left-handed opponents much less often, their ability to perceive, interpret, and react to these opponents’ movements is less developed. A number of experimental studies in which athletes watch video and assess the direction of an oncoming ball support the theory:
- Tennis players are better at predicting the direction and distance of shots made by a right-handed opponent than a left-handed opponent
- The outcome of balls struck by left-handed volleyball players’ actions was significantly less accurately predicted than the outcome of right-handed attacks;
- English Premier League goalkeepers are less adept at predicting the trajectory of balls kicked by left-footed penalty kickers.
More recent research reveals that left-handers are particularly overrepresented in sports where reaction times are very short. A 2017 review of professional ball sports found a very strong link between the response time for players in a game and the overrepresentation of left-handed players. Lefties appear at a normal rate in ball sports where players have relatively more time to react — badminton (12.6%), squash (8.7%), and tennis (13.9) – but are greatly overrepresented when reaction times are very short, such as cricket bowlers (21.2), table tennis (25.8%), and of course baseball pitchers (30.4%).
There is evidence that extensive training and practice can overcome this perceptual frequency effect in some sports. Still, the persistent success of lefties in such sports as fencing and table tennis indicates that in some sports, especially those with very short reaction times, no amount of training can erase the unfamiliarity effect. Moreover, in a team sport like baseball, extensive regular practice against left-handed opponents is not a realistic option for most young players. An exception who proves the rule was switch hitter Mickey Mantle, who as a boy hit frequently against his southpaw grandfather and went on to post a remarkable career OPS of 1.000 against left-handers (compared to “just” .965 against right-handers.)
Interestingly, the reason that scientists study left-right differences in sport is to test an evolutionary theory about the mysterious persistence of left-handedness in humans. Left-handedness is correlated with a number of negative health conditions, so natural selection should have largely removed this trait (which has genetic roots) long ago. One possible explanation is the “fighting hypothesis,” which holds that left-handed men have enjoyed an advantage in combat against opponents who were more used to fighting right-handed men. Since this advantage only obtains when lefties are relatively rare, left-handedness persists but stabilizes at a low frequency (about 10%). The dominance of left-handed men in interactive sports — considered a proxy for combat in earlier days – constitutes evidence in favor of the fighting hypothesis.
We are not evolutionary biologists, so we will not take sides in this debate. However, this body of research clearly supports the notion that hitters will have more trouble recognizing and responding effectively to pitches from a left-handed pitcher. For hitters, this effect may register as greater discomfort when facing a southpaw, or a (mis)perception that their pitches have greater movement. Indeed, the persistent myth of the “crafty” lefty who disrupts hitters with nasty pitch movement – though not substantiated by scientific measurements – likely has its roots in this lack of familiarity.
It may seem surprising that major league hitters would feel unfamiliar with southpaws’ delivery or pitch trajectories, when they face them almost 30% of the time. But remember, this incidence is much lower when a player is first learning to hit. In youth baseball the proportion must be much closer to the population average of 10%, as lefties are only selected for at more advanced levels of competition. If you still doubt that southpaws appear as novelties to opposing hitters, consider that Baseball-Reference lists no fewer than three dozen pitchers named “Lefty” in MLB history (and many more who carried the nickname informally), while the sport is still waiting for someone named “Righty” to take the mound.
We should acknowledge here that, after publishing an article based on this study, we learned that Ben Lindbergh advanced a similar theory years before us, also noting lefties’ inferior velocity, in an article for Baseball Prospectus. Now, let’s see if we can measure the impact of the unfamiliarity effect that Lindbergh and we have identified.
How Large Is Southpaws’ Advantage?
For this analysis, we define the “southpaw advantage” as the overall advantage a pitcher gains solely from being left-handed, as compared to a right-handed pitcher with equal pitch quality. If we can identify a subset of lefties who display similar underlying pitch quality as the right-handed population of pitchers, then any difference in their outcomes against hitters should reflect the southpaw advantage.
As we have seen, southpaws have pitched 28% of major league innings in recent years, despite representing just 10% of the male population. Assuming this disparity is a function of the southpaw advantage, what would we see in an alternative universe without that advantage? Well, if 10% of all pitchers were left-handed, those jobs would presumably be held by the best 36% of current lefties (10%/28%). And in that scenario, the other 90% could consist of today’s right-handers (72%) plus some weaker righties (18%) who replace the now-demoted lefties. So today’s righties would comprise the best 80% (72%/90%) of all righties in this world without a southpaw advantage.
Taking all that together, we hypothesize that only the best 30% of current lefties will have a pitch quality equal to the average current right-hander (80% of 36% – actually 29%, but we’re rounding). And a corollary of that proposition is that most of the remaining left-handers – who would not be in the majors without benefit of the unfamiliarity bonus — will display less underlying skill than even the weakest of today’s right-handers.
Does the data support this rather dramatic prediction? Returning to our sample of pitchers from 2007 through 2019, we sorted them into three groups for each pitching hand based on runs allowed per nine innings (Runs/9), giving us the top 30%, the middle 40 %, and the bottom 30%. When we say “best 30%,” we mean the total performance of the set of top left-handed pitchers whose innings pitched sum to 30% of the total. This is usually less than 30% of the actual pitchers, since the best pitchers tend to throw more innings than average. Note also that we have regressed each pitcher’s Runs/9 by adding 210 IP of league average performance (4.37), to account for the fact that each pitcher’s record contains some luck even with these relatively large samples. The table below reviews the average pitch velocities for each of these tiers of pitching performance:
| Runs/9 | Four-seam Velo | Sinker Velo | Cutter Velo | Curve Velo | |
|---|---|---|---|---|---|
| All Lefties | 4.34 | 91.8 | 90.7 | 86.6 | 76.7 |
| Top 30% | 3.75 | 93.1 | 91.7 | 87.9 | 77.3 |
| Middle 40% | 4.38 | 91.6 | 90.9 | 86.1 | 76.8 |
| Lowest 30% | 4.89 | 90.9 | 89.7 | 85.5 | 75.9 |
| Runs/9 | Four-seam Velo | Sinker Velo | Cutter Velo | Curve Velo | |
| All Righties | 4.37 | 93.3 | 92.0 | 88.7 | 78.6 |
| Top 30% | 3.80 | 93.8 | 92.4 | 89.2 | 79.2 |
| Middle 40% | 4.39 | 93.3 | 92.1 | 89.2 | 78.6 |
| Lowest 30% | 4.92 | 92.7 | 91.5 | 87.6 | 77.8 |
There is a lot of important information here. First, we see that left-handers deliver markedly lower pitch quality than right-handers at every performance level, despite equal outcomes against hitters (runs allowed). It is also apparent that more successful pitchers tend to throw at higher velocities. Velocity is by no means the entire story — the difference in runs allowed between adjacent tiers is larger than velocity alone can explain — but it’s clearly a strong signal of differences in talent.
Overall, the velocity data fits our hypothesis quite well. The best southpaws (top 30%) throw their four-seam fastballs and sinkers at about the same velocity as the average or median right-handed pitcher (actually a tick slower). The gap is a bit larger on changeups and curveballs, with the best lefties’ velocity lower than the average righties’.
Note that these top lefties allow just 3.75 runs per 9 innings (regressed), fully 0.62 better than the average righty. To be conservative, we’ll ignore that the top lefties are actually a little worse than the righties they’re being compared against, and we’ll round down. Our best estimate of the overall southpaw advantage, based on this data, is 0.60 runs per 9 innings:
| Four-seam Velo | Sinker Velo | Cutter Velo | Curve Velo | Runs/9 | |
|---|---|---|---|---|---|
| Top 30% of lefties | 93.1 | 91.7 | 87.9 | 77.3 | 3.75 |
| All righties | 93.3 | 92.0 | 88.7 | 78.6 | 4.37 |
As theorized, the bottom 70% of lefties throw slower pitches than even the weakest group of righties on each pitch type. These are mainly the lefties who would not be in the majors were it not for the unfamiliarity effect. If we compare the middle 40% of lefties to the bottom tier of righties, we see substantially lower velocity on fastballs (91.6 vs. 92.7), sinkers (90.9 vs. 91.5), cutters (86.1 vs. 87.6) and curves 76.8 vs. 77.8). The bottom performance tier of lefties, unsurprisingly, trails even further behind.
For this analysis we have assumed that the southpaw advantage is a constant for all left-handed pitchers. In reality, it must vary at least somewhat by pitcher, and perhaps by the type of pitcher. For example, it appears that the lefty/righty gap in velocity is larger at lower performance levels than among the best pitchers (see table). Since each tier performs equally in allowing runs, that suggests the possibility that the unfamiliarity effect may convey a relatively greater benefit to left-handed pitchers with relatively weaker pitch quality skills. This seems to us a potentially fruitful area for further research:
| Four-seam Gap | Sinker Gap | Cutter Gap | Curve Gap | |
|---|---|---|---|---|
| Top 30% | -0.7 | -0.7 | -1.3 | -1.9 |
| Middle 40% | -1.7 | -1.2 | -3.1 | -1.8 |
| Lowest 30% | -1.8 | -1.8 | -2.1 | -1.9 |
Linear Regression
Thus far, we have measured pitchers’ underlying skill solely in terms of velocity. We know, however, that other factors also substantially affect pitcher performance. If we turn to Statcast data, we can create a fuller picture of overall pitch quality. For seven different pitch types, Savant gives four parameters for every pitcher/season from 2017 to 2019: average velocity, average spin, average horizontal movement, and average vertical movement. (The two movement variables are normalized for velocity and big-league average.)
We ran a regression using those 28 “skill” variables to try to predict pitchers’ xwOBA allowed. The predicted xwOBA allowed for each pitcher is essentially a total pitch quality score, combining his performance on all the speed and movement variables, weighted by their relative importance. This allows us to make a much more complete comparison of the pitch quality for different performance tiers of left- and right-handed pitchers. The following table compares expected xwOBA allowed, converted into Runs/9 for ease of comparison, for the same three performance-based tiers we used earlier (using pitcher seasons from 2017-2019):
| Runs/9 Rank | LHP | RHP |
|---|---|---|
| All | 4.41 | 4.15 |
| Top 30% | 4.20 | 3.95 |
| Mid 40% | 4.44 | 4.15 |
| Low 30% | 4.58 | 4.33 |
The results closely mirror our earlier velocity-based analysis. Once again, we find that the top 30% of left-handed pitchers have virtually the same skills (projected 4.20 Runs/9) as right-handed pitchers overall (4.15). And again we see that the remaining 70% of southpaws have lesser pitch quality than the weakest right-handers. This provides strong confirmation of our core thesis: the top tier of lefties are peers of the average righties in terms of pitch quality, with their disparate outcomes a function of the unfamiliarity factor.
We then ran a second regression, this time adding a dummy variable to represent whether the pitcher is left-handed, as an alternative method for estimating the southpaw advantage (the gap between performance and pitch quality). The result: the left-handed advantage came out to 0.30 runs per nine innings, about half the size of our previous estimate of 0.60 runs.
We believe the earlier, larger estimate of 0.60 is much closer to the mark, for two reasons.
First, the regression includes only the 28 talents provided by the Savant data. Importantly, those are the only dimensions we are able to measure directly. Given that pitchers are selected in a performance-based competition for jobs, it follows that right-handers should be stronger on every pitching skill that yields success on the mound, such as command, deception, and pitch sequencing. However, we haven’t measured those abilities here. The regression simply assumes that left- and right-handed pitchers are equal in all dimensions not explicitly controlled for, so it implicitly overestimates the other talents of lefties relative to righties, causing the southpaw advantage to be underestimated.
To see how that happens in practice, here are the regression estimates for the southpaw advantage when we don’t use all the talents, but just add one category at a time:
- 0.17 R/9 – velocity only
- 0.24 R/9 – velocity and spin only
- 0.30 R/9 – velocity, spin, vertical and horizontal break.
It seems reasonable to assume that if we had access to measures of other dimensions of pitcher talent, the calculated advantage would increase further.
Second, southpaws have an inherent advantage over righties when it comes to holding runners close to first base, reducing stolen base attempts and presumably limiting base runner advancement on hits. It is beyond the scope of this paper to measure precisely the impact of holding runners at first. However, we do know that lefties have a small advantage of .0043 in their strand rate (2007-2019), meaning that fewer of their allowed baserunners ultimately score, which should largely capture this benefit. That translates into an advantage of about 0.05 in runs allowed per 9 innings, which means the regression coefficient will be 0.05 too low.
Third, we believe a selective sampling issue may be reducing the regression estimate. Consider a lefty who allows 4.40 runs per game, but would allow 5.00 based on pitch quality alone. The regression cannot compare him to right-handers with identical throwing ability, because most of those pitchers are in Triple-A. The only righties in the majors with similar observable Statcast ratings are those who have some additional, unmeasured skills (perhaps superior command) that allow them to perform better than indicated by Statcast measurements alone. The regression mistakenly sees these two pitchers as equals (except for their pitching hand), and, as a result, the advantage attributed to throwing left-handed gets understated.
Overall, the regression coefficient is short of our estimate by 0.25 R/9; we believe this is a combination of unmeasured pitcher data and selective omission of the weakest right-handers.
Breaking down the Southpaw Advantage
Is it really plausible that major league southpaws could be 0.60 runs per nine innings worse than their right-handed teammates, in terms of underlying pitching skill? We believe it is.
Research in 2010 by Mike Fast demonstrated that a loss of 1 mph of fastball velocity increases runs allowed by 0.28 runs per nine innings on average. As such, the fastball velocity differences that we found alone would create a handedness skill gap of about 0.40 runs. In addition to velocity, righties are also superior when it comes to spin and many forms of pitch break, which must further widen the gap. As further research makes more skill dimensions available as data, our estimate of 0.60 runs per game could even prove conservative.
As large as this 0.60 skills/outcomes disparity is, the unfamiliarity advantage enjoyed by lefty pitchers is even larger still. Remember, our measurement represents the combined net impact of the unfamiliarity bonus (a positive for southpaws), the platoon penalty from facing more opposite-hand hitters (a negative), and the advantage of being able to better hold baserunners (positive). As mentioned earlier, we estimate that the platoon penalty increases Runs/9 of lefties by about 0.20 relative to right-handed pitchers, and the baserunner advantage decreases Runs/9 by 0.05 relative to righties.
Taking all of these factors into account, we believe that left-handed pitchers’ “unfamiliarity bonus” is a remarkable 0.75 runs per nine innings:
| Factor | Impact on Runs/9 (vs. RHP) |
|---|---|
| Pitch quality | 0.60 |
| Platoon penalty | 0.20 |
| Holding baserunners | -0.05 |
| Unfamiliarity bonus | -0.75 |
| TOTAL | 0.00 (equal outcomes with RHPs) |
Interestingly, the fact that the unfamiliarity bonus is greater in magnitude than lefties’ pitch quality deficit helps explain a long-noted, but never satisfactorily explained, aspect of batter platoon splits. Left-handed hitters have a much larger platoon split (115 points of OPS) than right-handed hitters (69 points). Yet there is no obvious reason why facing a same-handed pitcher should be any more challenging for lefty hitters. These figures provide at least some of the explanation. If we remove the impact of the platoon penalty, we see that left-handed pitchers are actually a bit better than right-handers in platoon-neutral performance, by about 0.20 Runs/9. That means left-handed hitters face better pitchers when the pitcher has the platoon edge, enlarging the batter’s platoon split; the effect is reversed for right-handed hitters, who face better pitchers when hitting with the platoon advantage, narrowing their platoon split:
| Facing RHP | Facing LHP | Platoon Split | |
|---|---|---|---|
| RH Hitters | .719 | .788 | .069 |
| LH Hitters | .816 | .701 | .115 |
Conclusion
It seems fair to say that the southpaw advantage has shaped the game we know in fundamental ways. Obviously, an advantage of 0.60 in runs allowed per nine innings constitutes an enormous performance difference in the major leagues. That’s the difference between excellent (David Price, 3.63) and merely good (Chris Archer, 4.28), or between an average starting pitcher and a fifth starter struggling to keep a spot in the rotation. At the team level, giving up .60 fewer runs a game would turn an average 81-win team into a 92-win postseason contender.
Looking at the bigger picture, without southpaws’ hidden advantage, about two out of three left-handed starters would likely be toiling in the bullpen, or in the minor leagues. It’s safe to say that Chris Sale and Clayton Kershaw would still have jobs, but they would probably not dominate hitters to the same degree. Pitchers like Jon Lester and Dallas Keuchel would likely be average starters at best, rather than stars. You would be able to count all the lefty pitchers in the Hall of Fame on your fingers, and every few years we might debate whether a southpaw will ever win the Cy Young Award again.
Bullpens would also look very different. Nearly all high-leverage relievers would throw from the right side, while the occasional Aroldis Chapman or Billy Wagner would stand out as oddities. Indeed, lefty relievers would be sorely needed in this alternative timeline, because if batters rarely had to face a southpaw, then lefty hitters would enjoy an even bigger advantage than in the existing game. A large majority of hitters would surely bat left-handed or switch hit. Ruth, Williams, Musial, and Bonds would tower even higher in baseball history, while Hornsby, Mays, Schmidt, and Trout ranked a bit lower in the pantheon.
Back in our world, though, lefties get to enjoy a hidden advantage perhaps more powerful than any PED. Southpaws have long had to endure being stereotyped as “weird” or “wacky,” but they should be grateful for their perceived strangeness. As it turns out, there really is “one weird trick” that vastly improves pitching performance: being born left-handed.