So what *did* cause the home run explosion?

In a recent post, I suggested that the recent increase in home run rates might be due to players realizing that if they bulk up, some of their fly balls turn into home runs, and that might be the easiest way to increase offense.

But commenters here, and at BTF, pointed out that the increase pretty much took place instantly, over the two year period 1993-1994. And it's not likely that players bulked up instantly, so that theory is out.

So what was it that made home runs suddenly increase so much? Tangotiger pointed out this study at highboskage.com, which suggests it was a juiced ball. Which makes sense to me, except – what about strikeouts? It wasn't just home runs that increased, but also Ks.

On his blog, Tangotiger gives us several rates of increase from 1992-94:

pre93 post93
----- -----
0.285 0.298 BABIP
0.769 0.739 contact
0.292 0.336 XBH/H
0.318 0.351 HR/XBH
0.147 0.167 HR/K
0.530 0.487 BB/K
0.217 0.254 K/outs

(glossary: BABIP = batting average on balls in play. Contact = (roughly) non-K/AB. XBH = extra base hits.)

And, of course, home runs per game, up 43%:

0.721 1.033 HR/G

A juiced ball could easily explain the increase in home runs, but what about the increase in strikeouts? How would a juiced ball cause strikeouts? It's possible that the strike zone could have been enlarged at the same time, but walks were also up those years.

One possibility is that teams suddenly decided to go for more home runs at the expense of more strikeouts. But why would that have happened all of a sudden in 1993, continued into 1994, and then levelled off until today? And it would take a lot of teams to do that to make a 43% difference in home runs. Why would they all do it at once?

A possible, but implausible, hypothesis is this:

Teams suddenly discovered that the ball would be livelier in 1993. Either the commissioner told them, or they figured it out early in spring training. They informed their power hitters that deep fly balls were likely to leave the park this year. The hitters responded by swinging for the fences more, thus increasing their strikeouts at the same time as their home runs; pitchers responded by pitching more carefully, thus also increasing walks. In 1994, the trend continued; and, furthermore, singles hitters found themselves out of a job because what were previously marginal power hitters now were able to hit well enough to take their jobs. That pushed the effect forward for another year. But by 1995, all the adjustments had been made, and offense levelled off.

If that were true, we'd expect to see more fly balls hit. But we don't. I figured the FB/(FB+GB) ratio for the years 1992-1994:

1992 ... 41.6%
1993 ... 41.1%
1994 ... 40.9%

(Technical note: a ground ball was any Retrosheet play with /G. A fly ball was any play with /F, or a home run. These numbers might be very slightly off because I averaged all the monthly figures without weighting by PA.)

My programming, or the Retrosheet data, could be off, but it does look like that in those years, fly balls went *down*, not up. So what's going on?

Another interesting thing is that the changes didn't occur gradually, month-over-month – almost all the change was between seasons:

Apr 1992: 9 HR per 500 PA
Sep 1992: 9 HR per 500 PA

Apr 1993: 11 HR per 500 PA
Sep 1993: 11 HR per 500 PA

Apr 1994: 14 HR per 500 PA
Aug 1994: 13 HR per 500 PA

What is also apparent here is that there were two separate increases, one to start 1993, and another to start 1994. And strikeouts show the same pattern – jumps between seasons, rather than within them.

So it wasn't that it was slowly and gradually dawning on hitters that the ball was juiced: any factors responsible appear to have been in place at the beginning of 1993 and 1994.

I'm at a loss. What could be going on that home runs, strikeouts, and walks would all increase at the same time?