Doctoring the Numbers: Where Have All the Southpaws Gone?

One of the most striking changes to the landscape of major-league baseball
has been the disappearance of the left-handed starting pitcher. In the
American League, left-handers made fewer than 20% of all starts last year,
allowing teams like the A’s to take advantage by stacking their lineup with
left-handed hitters.

How dramatic is the decline? Here is a list of the percentage of starts
made by left-handers in the AL over the last five years:

Year  Lg   LHP   RHP     LH%

1999  AL   449  1816   19.8%
1998  AL   608  1674   26.6%
1997  AL   643  1621   28.4%
1996  AL   651  1615   28.7%
1995  AL   622  1403   30.7%

Teams were more than 35% less likely to face a left-handed starter in the
AL in 1999 than they were in 1995. But compare that to the National League:

Year  Lg   LHP   RHP     LH%

1999  NL   697  1894   26.9%
1998  NL   670  1940   25.7%
1997  NL   548  1720   24.2%
1996  NL   524  1744   23.1%
1995  NL   566  1448   28.1%

Between 1995 and 1996, the percentage of left-handers dropped
precipitously, but southpaws have actually made a higher percentage
of starts each year since.

Is the number of left-handed pitchers at a historic low in the American
League? Not even close. The 1999 AL ranks just 24th on the list of the most
right-handed leagues this century. The four most right-handed leagues were
the 1902-03-04-05 NL, a span of time in which just 661 of 4679 starts
(14.1%) were made by left-handers. (In the AL during the same four years,
lefties made 25.5% of starts.)

For the 19th century as a whole, left-handers made just 13.2% of all starts.

But while left-handers are not at their lowest point ever, they are rarer
than they have been in more than half a century. Here are the overall
percentage of starts made by left-handers, broken down in five-year
increments:

Years       LH%       Years     LH%

1900-04   20.4%     1950-54   32.7%
1905-09   22.8%     1955-59   26.3%
1910-14   23.7%     1960-64   30.8%
1915-19   27.1%     1965-69   32.0%
1920-24   25.2%     1970-74   32.0%
1925-29   27.5%     1975-79   31.8%
1930-34   25.3%     1980-84   31.1%
1935-39   21.5%     1985-89   32.4%
1940-44   21.7%     1990-94   31.2%
1945-49   28.6%     1995-99   26.2%

The 26.2% of starts made by left-handers between 1995 and 1999 is the
lowest total in any half-decade since the 1930s. This comes after a 30-year
stretch in which the frequency of starts by left-handers remained
remarkably consistent between 31.1 and 32.4 percent.

In what leagues were left-handed starters most common?

Year  Lg   LHP   RHP     LH%

1949  NL   506   738   40.7%
1983  AL   867  1403   38.2%
1950  AL   471   769   38.0%
1990  NL   724  1220   37.2%
1992  NL   721  1223   37.1%

In addition to 1990 and 1992, the 1991 NL (36.2%) ranks ninth all-time. As
much as the current dearth of left-handers, it is the relative abundance of
them less than ten years ago that makes their decline seem all the more
dramatic.

The five most left-handed teams since 1900:

Year   Team            GS by LHP

1983   New York (AL)         127
1975   Chicago (AL)          124
1974   Baltimore             116
1979   Chicago (AL)          116
1982   Kansas City           113

For whatever reason, nine of the ten most left-handed teams played between
1974 and 1983, the lone exception being the 1965 Dodgers. It’s not just the
longer schedule; if you rank teams by percentage of starts made by lefties,
only one earlier team (the 1951 Red Sox) breaks the top ten. (The 1981
Yankees, with 85 of 107 starts made by left-handers, would actually rank
first.)

Perhaps the greater question is, "how much difference does it
make?" You frequently hear it said that teams can’t win without a
left-handed starter. No doubt, this stems from the Yankees’ historic
dominance and the number of great left-handed starters that have toiled for
them. But keep in mind that Yankee Stadium has always been a very tough
park for right-handed hitters, especially power hitters, particularly
before it was redesigned after the 1972 season. Playing in a park that
favors left-handed pitching, it isn’t surprising that they’ve had so much
success with southpaws.

Let’s look at the correlation between the percentage of starts made by a
left-hander and a team’s overall winning percentage:

LHP%      Teams    WPct.    Yankees    WPct.      Others    WPct.

>=50%       162    .514          16    .556          146     .510
40-50%      272    .511          12    .537          260     .510
30-40%      407    .509          24    .602          383     .504
20-30%      497    .493          20    .591          477     .489
10-20%      333    .496          17    .537          316     .494
<10%        257    .484           8    .533          249     .483

The first two columns show that there is indeed a very strong correlation
between the percentage of starts made by left-handers and the team's
overall winning percentage. On average, teams who give at least half their
team's starts to left-handers have a winning percentage 30 points higher
than teams who give less than 10% of their starts to lefties, an advantage
of five games over a 162-game season.

But, as the next two columns show, nearly 10% of the 162 most left-handed
teams played in the Bronx, while barely 3% of the 257 most right-handed
teams were Yankees. The Yankees themselves show little correlation between
left-handed pitching and success, but the team's overall success has skewed
the numbers slightly. When the Yankees are ignored, we can see that while
left-handed pitching still correlates with success, the difference is only
about three-quarters as great. Still, an advantage of three or four games a
season is not something to be trifled with.

Rany Jazayerli, M.D., can be reached at ranyj@baseballprospectus.com.