The immediate reaction most of us had to hearing of Robbie Cano's entry into the Home Run Derby (and now Nick Swisher's) was that of consternation. We've heard horror stories about it messing up swings and making hitters pull-happy, especially after Bobby Abreu's 2005 season. Abreu hit 18 homers in the first half and six in the second. Many attributed his let-down to the Derby.
So I set out to determine whether the HR Derby truly affects its participants. I took every Derby participant from 2000 on (79 in total*) and compared their first half to their second. The results were not entirely surprising:
Overall, hitters fared worse after the HR Derby. The average participant entered the All-Star break with a .993 OPS. The average second half OPS was .951, a drop of .042. Not huge, but not insignificant either.
A few, though, fell off a cliff. In 2001, Todd Helton entered the All-Star break with a 1.252 OPS. His second half OPS (.977) was still excellent, but represented a .275 drop off. Some others include:
Paul Konerko, 2002: .949 vs. .734
Jim Edmonds, 2003: 1.066 vs. .864
Hank Blalock, 2004: .941 vs. .743
Justin Morneau, 2007: .944 vs. .702
Dan Uggla, 2008: .978 vs. .739
Lance Berkman, 2008: 1.096 vs. .821
Brandon Inge, 2009: .876 vs. .542
However, not every player declined. Of the 79 seasons/hitters, 34 (43%) actually improved in the second half, but there were only two extreme examples:
Sammy Sosa, 2000: .962 vs. 1.138
Ryan Howard, 2006: .923 vs. 1.259
Overall, most hitters declined, a few "fell off a cliff," and the vast majority of 'improvers' only did so moderately.
Before coming to a definitive conclusion, I needed to find out whether hitters in general declined in the second half (perhaps due to fatigue). They did not. The average major league OPS (from 2000-'09) entering the break was .760. The second half average was .759. (The exact difference is -.00092.) That slight difference is essentially meaningless, and may have more to do with September call-ups getting a chance than any kind of decline among ML 'regulars'.
So - our initial reaction is backed up by evidence. But don't judge Swisher's decision just yet, because (1) even if he does decline, it probably won't be all that much, and (2) Swisher is traditionally a better first half hitter (so a slight second half decline wouldn't be out of the ordinary).
My advice is: Try to enjoy the Derby and pray Swisher doesn't get hurt.
*79 rather than 80 because Ivan Rodriguez, a participant in the 2000 Derby, only played in 10 games the second half, so I discounted that season.