The rate of home runs hit per balls in play in in the first half of this MLB season is the second-highest of all-time, with only the first half of the 2000 season being higher.
The only thing as ubiquitous as Pokemons these days are baseballs flying out of Major League ballparks. Going back to 1920, the start of the lively ball era, only the first half of the 2000 season has seen more balls in play land for home runs than the first half of the 2016 season. I thought something strange was happening this year, so I went to the record book. I found the rate of home runs per balls in play for the first half of each MLB season going back to 1920. I’m using the first half of each season to compare apples to apples, since we’re not yet through the entire 2016 season. I’m also using the rate of home runs per balls in play because I feel that’s the best metric to isolate the rate at which home runs are being hit. Home runs per balls in play is simple. It excludes walks, strikeouts, HBP, and sacrifice bunts, but not sacrifice flies. If a ball is put into play, it counts.
This season’s first half rate of HR/BIP is 4.4% and has only been bested once since 1920. The one season with a higher rate is the first half of the 2000 season, when the HR/BIP rate was 4.5%. In fact, there have only been three seasons since 1920 in which the rate of HR/BIP was as high as 4% in the first half—1999, 2000, and 2001. Those three seasons came during a stretch of high offense seasons that saw runs per game jump to levels not seen since the depression-era 1930s.
The history of HR/BIP in MLB is an interesting one. Before the advent of the lively ball in the early 20s, MLB hitters saw roughly 0.5% of their balls in play go for home runs. The introduction of a livelier ball pushed that number up over 1.0% very quickly, and it further increased up to around 2.0% in the mid 1930s.
During World War II, many MLB players served in the military. Also, the U.S. banned the use of rubber for non war-related goods, including for baseballs. In 1943, MLB used rubberlike shells made of balata instead of real rubber and the rate of HR/BIP dropped precipitously, to 1.1%. It increased as players returned to MLB after serving in the military and the ball was restored to its previous composition. By 1947, HR/BIP was back up to pre-war levels.
The first half of the 1955 season was the first time the HR/BIP rose over 3.0% and it generally stayed at or near that level through the rest of the 50s and most of the 60s. Then, suddenly, offense plummeted in 1968, as did the first half rate of HR/BIP (2.3%, the lowest level since 1949). The powers that be in Major League Baseball lowered the mound and reinstated the 1961 strike zone and the HR/BIP rate jumped up to 2.9% in 1969.
Baseball in the 1970s and 1980s generally had first half HR/BIP rates between 2.5% and 3.0%, except for the bizarre 1987 season, when it shot up to 3.9%. That “blip” lasted just that one season. The very next year, the first half HR/BIP rate dropped to 2.7%. The next big jump came in the 1990s. The rate rose from 2.6% in 1992 to 3.0% in 1993 to 3.6% in 1994. This was the advent of a new home run hitting era in baseball that peaked in 1999 (4.0%), 2000 (4.5%), and 2001 (4.1%). Many people blame PED use for the high-offense era from 1994 to the early 2000s. It’s possible that PED was the driving factor, but the composition of the ball could have played a role as well.
Next: Outlier Seasons