There are plenty of famous "curses" in sports: the Madden curse, the Sports Illustrated curse, and countless more. Around this time each year, another one starts to come up: the "Home Run Derby curse" - that is, the tendency for players in the Derby to perform worse in the second half of the season than they did in the first. With Manny Machado participating this year, a small number of O's fans will inevitably be worried that swinging for the fences in a meaningless competition will mess with his swing or his timing, and that a slump may follow.
On the surface, it's not completely ridiculous. Players are certainly taking different kinds of swings in the Derby than they are in real games. Just like my coaches used to tell me never to play golf during the season because the golf swing can mess up your baseball swing, it stands to reason that an exaggerated power swing could do that too. Additionally, some think that resting instead of participating would help a player out, as if the 50ish swings he might take that night will affect his energy level in September. So there are some (flimsy) reasons why this might be true, but is it a real phenomenon? Is Manny screwed unless he pulls out of the event at the last minute? In a word, no.
On one hand, players who make the Derby tend to do worse in the second half of the season. That's not up for debate - a 2010 Baseball Prospectus article showed that on average, Derby participants had a drop in OPS of about .045 and a drop of HR% of about 0.65% after the break. But that doesn't mean that this decline in production had anything to do with a few swings during a Saturday night exhibition event. Instead, it's probably just explained by regression to the mean. The players who are picked for this event tend to be guys having really good seasons, sometimes by far the best of their career. If you pick any group of players having career years through July and look at what they do from August to September, you'll almost always see a decline.
In fact, the same article looked at this too. They took all of the players who participated in the Derby during a season when they posted their career-best first half OPS. Then they looked at those same players' best half-seasons when they didn't take part in the Home Run Derby. Surprise! They had almost an identical drop off: .044 in OPS and 0.65% in homer rate.
Just for kicks, even though this theory has been pretty much debunked, let's look at past Orioles participants, and what they did before and after the break.
Eddie Murray (1985) - Eddie was actually better in the second half of 1985, increasing his OPS from a very good .819 to a ridiculous .999, and hitting five more home runs (18 vs. 13).
Cal Ripken (1985, 1991, 1992) - After his first of three appearances in the Derby, Cal was the exact same player in the second half of the 1985 season (.282/.352/.465 vs. .282/.342/.472). The 1991 season was a different story, mainly because of how insanely good he was before the break - I have a feeling his .348/.405/.596 line would not have continued regardless of what he did that night in July. He still was very good in the second half OPSing .881 and hitting 16 more home runs to bring his total to 34. If you're looking for examples of the "curse", 1992 would be one of them. Cal was picked for the Derby that year mainly because of his prior season - he was batting a mediocre .262/.347/.397 with ten home runs at the halfway point. He performed even worse in the second half, though, with an .624 OPS and only four more bombs.
Brady Anderson (1996, 1997) - Brady's 1996 season was similar to Ripken's 1991. He wasn't as good in the second half, but doing so would have been almost impossible after slugging .678 and hitting 30 home runs in his first 79 games. He still played very well in the second half, going .298/.388/.592 with another 20 homers. We all know Brady didn't come close to matching that production in 1997, but his offense was spread pretty evenly between the beginning (.300/.421/.444) and end (.274/.361/.498) of the season. He actually hit four more home runs in the second half than he did in the first (11 vs. 7)
Rafael Palmeiro (1998) - Even though '98 was disappointing for the O's, Raffy was an elite hitter the entire time. He OPSed .959 in the first half and .928 in the second.
B.J. Surhoff (1999) - B.J. was another case of a player having a career year and coming back to earth in the second half. He batted .332 with 20 home runs in the first half, and his second half was more in line with his career numbers (.277 AVG, .737 OPS, 8 homers). Believers in the curse will call this a classic example; I call it a player having an incredible half-season and then regressing to the mean.
Miguel Tejada (2004, 2006) - Miggy won the whole thing in 2004, hitting a then-record 15 homers in the semi-final. This was also the season where he famously drove in 150 runs, splitting them evenly with 75 RBIs in each half. He batted exactly .311 in both halves and actually increased his power by slugging 60 points higher after the break. He followed that trend again in 2006 - he hit .350(!) after the break and increased his OPS by 15 points.
Chris Davis (2013) - In this case, it was simply not feasible for Davis to keep up what he was doing in the first half, with 37 homers through 95 games and a 1.109 OPS. The rest of the season he "only" OPSed .854 and hit 16 more home runs.
Adam Jones (2014) - AJ did tail off a bit in the second half of the season, with a .735 OPS (vs. .810) and a 50-point drop in batting average. To be fair, he did play in 161 games, so the fatigue from that kind of workload may have played a role in that decline.
As Baseball Prospectus and others have pointed out, the Derby curse (like most sports curses) is probably not a real thing. Beyond that, O's players have seemed to buck that trend even more than the rest of MLB. Be concerned about Manny if you want, but my advice is this: watch the Derby on Saturday, root for Manny, and enjoy it. If he doesn't keep up his incredible level of play so far this year, the Home Run Derby won't have anything to do with it.