Increase in Home Run Productivity at the Major League Level

One hundred years ago - 1907 - the percentage of home runs per 100 at bats was 0.305, according to baseball-almanac. In 2005 - the last year posted there - it was 3.016. Essentially what that means is that in the major leagues, in 1907 it would take 1000 at bats to equal the home run production of 100 at bats in 2005. If you wish to compare the nice round 100 years figure, the home run production in 1905 was 0.413 per 100 at bats, making my 1000 at bat figure a little less. A ten fold increase in home run production in about 100 years.

1911 saw a small spike up, then progressively down through 1918. 1919 saw a bump back up and in 1921 the percentage hit 1.1 for the first time in baseball history. This corresponds to the end of the dead ball era and the decision of major league baseball to ban trick pitches (such as the spit ball) in favor of more offense. In 1920, the rules were changed to allow a batter a home run in the last of the 9^th if the winning run was on base when it was hit.

The trend toward more home runs continues through the 1940s with a slight decline through the Second World War, but going below that 1.1 number once - in 1943 it hit 1.068. After 1943 the number steadily rises through the 1949 season when it hit over 2 for the first time and where it stays until the 1968 season when it dips below 2. In 1969 - partially responding to the low 1968 production - baseball lowered the pitchers mound by 10 inches. Perhaps more important to increased home run production in 1969 - it jumped by .5 percentage points from 1968 - was the shrinking of the strike zone to be from the batters armpits to his knees. The rate continued in the 2 per hundred range until 1976 when it dipped to just under 1.7.

The next time the totals dipped below 2 per hundred was in 1981 - a strike year and the first time a work stoppage affected a significant portion of the season. The next time a work stoppage affected the season (1994-1995) home run totals climbed to and remained at an all time high of over 2.9. The subsequent season, 1996, saw the number top 3 home runs per 100 at bats for the first time, where it has remained since with the exception of 1997. What makes the McGuire/Sosa 1998 chase of most home runs in a season particularly interesting is that the National League totals per 100 at bats was lower than the AL at 2.89.

There can be a lot of explanations for the increase - from expansion (and therefore a dilution of the talent pool) or conversely, the increased international presence and recruiting of the talent of African American players and later to Latino and Asian players. There has also been the addition of a specialized hitter in the American League (the "Designated hitter" or "DH"). Since the introduction of the DH, the AL has generally held a higher home run percentage than the National League with a few exceptions.

The composition of the bat itself has undergone significant changes over the years as well - from hickory wood, to ash, to ash/resin composites, to maple. The parks or stadia have become increasingly tailored to encourage home run production with buildings that while accommodate more spectators, create less outfield space.

The baseball season has expanded by 8 games perhaps giving more opportunity for batters to hit against pitchers tired from a long season - of course over the last 20 years or so, pitchers have become increasingly specialized in their respective roles would should argue against an increase. Spring training has also become more regimented. In the last 100 years, there have been changes in the definition of "at bat" which could account for some increase to the proportion of home runs hit per at bat over the course of time. Bases-on-balls or "walks" are not counted against a batters "at bats" and the rule on sacrifice flies has come and gone several times which would/could affect the counts. Perhaps the increased awareness of fitness and improved training regimens have increased productivity as well. Since this performance has been more highly compensated as a result of increasing strength of collective bargaining, more players are pursuing more aggressive training and sought more advantage in increasing production, which could include the unspoken steroid/performance enhancing drugs issue.

In sum, there is clearly not one cause, but a cumulative effect of several causes over the last 100 plus years. Rules changes to enhance offense - from eliminating "spitballs" in the 1920's to lowering the mound in the late 1960's to creation of the Designated Hitter have changed how the game is played; the composition of the bats - always with an eye to more production - has increased production; the advent of performance enhancing substances has likely increased individual production illegally and increased attention to physical fitness has likely increased production legally; more hitter friendly parks and in the increased number of cities where professional baseball is played has to have had an effect.