Over the past few years, it's felt like many players in MLB slug out around the same amount of home runs, yet the number of runs batted in decrease from year to year. For example (and this is complete generalization), back in 2005-2008 when a guy would hit 33 home runs, they usually also brought in at least 95 runs. While there is an element of randomness to it because it depends on what kind of players are batting before you in the lineup, there is an unusual trend that has occurred.
Here are graphs that show the HR-to-RBI ratios of the Top 100 players (ranked in the Home Run statistic), one for each year from 2005 to 2013 (June 11th). Note: The last numbers in each graph's title show the minimum and maximum numbers of home runs in each Top 100 list. Please click on each one to enlarge for a clearer image.
You'll see a defined function in the upper-right corner of every graph. That's the function that represents the graph. Besides multiplying the number of home runs (x) by a number between 1 and 2, a constant is added. The constants are, in order from 2005 to 2013: 40.68, 49.379, 48.044, 42.962, 50.421, 45.132, 35.527, 38.371, and 15.44. That is a key number in looking at this trend. That constant has increased by 9, decreased by 1, increased by 2, increased by 1, decreased by 5, decreased by 10, increased by 3, and for 2013 has decreased by 23 (although 15.44 is sure to change as hitters are sure to drive in more runs by non-HR methods). To take a bigger, overall look, I created graphs from 2005 to 2013 that look at the HR-to-RBI ratios by team. Please click on each one to enlarge it to a bigger and clearer picture.
The constant number in each of the functions follows the same pattern of spiky ups and downs as well.
Using each of the equations from the individual ratio graphs, I created a table that shows what the typical HR-to-RBI counts would be for each year between 2005 and 2013 using the home run counts of 15, 18, 20, 23, 25, 30, 35, 40, and 45. I made a graph to go with it, however it is hard to follow, so I'll post the x and y table of values as well.
For all hypothetical home run counts, the numbers of runs batted in in the past two years have been an all-time low. For 33 home runs, the expected RBI count of the past two years is slightly below 100, while for 2005-2010 it was at a decent number over 100. So what's to blame for this? Bad clutch hitting? Poor on-base percentages in general? Some can definitely be attributed to the "Home Run, Strikeout, or Walk" mentality of some major league teams, including the Braves.
The average OBP has decreased from year to year lately. From 2005 to 2013: .330, .337, .336, .333, .333, .325, .321, .319, .318. Walk rates have also decreased lately, going from 8.9% in 2009, to 8.5% in 2010, to 8.1% in 2011, to 8.0% in 2012, and to 7.9% in 2013. Strikeout rates have increased lately. From 2005 to 2013: 16.4%, 16.8%, 17.1%, 17.5%, 18.0%, 18.5%, 18.6%, 19.8%, and 19.9%.