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Poking around the internet, I found this:

A company that uses computer imaging claims baseballs had a larger rubberized core and a synthetic rubber ring in 1998, including the ball Mark McGwire hit for his 70th homer.


"Examining the CT images of Mark McGwire's 70th home run ball one can clearly see the synthetic ring around the core -- or 'pill' -- of the baseball," UMS president David Zavagno said. "While Mark McGwire may or may not have used illegal steroids, the evidence shows his ball -- under the governing body of the league -- was juiced."

I submit for your consideration:

(click image for larger size)

ISO is isolated slugging (SLG minus AVG) and HR/10AB is the number of home runs per ten at-bats.  Each data series is listed for the AL and the NL.  Isolated power and home run rate both measure how much power

Now, look at that graph and tell me when the ball was juiced.  Doesn't look like 1997, huh?

I see two likely possibilities:

  1. There was nothing special about the ball in 1997.
  2. The ball actually was changed in 1997, but it didn't make any real difference on how players were able to hit home runs.
The only way to get around these possibilities is to start inventing conspiracy theories about the league using different balls for games with McGwire and/or Sosa.

It's clear that there's a shift in offensive levels, but that shift happened from about 1992 to 1994.  It's unclear exactly why this shift occurred, though it's probably some combination of stronger players, different ballparks, and possible equipment changes.  Amongst all of those, I'd say that the players made the biggest difference, but I can't really back that up.

If I was to point to one year in this set and say "something weird was happening this year," it would maybe be the spike from '86 to '87 or the dip from '87 to '88.  But I'm guessing even those were probably just fluke years.

Anyway, whether or not these guys did all of their fancy imaging to determine that the ball was different in '97, there was no league-wide trend of hitting for more power, so any claim that juiced baseballs helped McGwire hit more HR in '97 has little or no basis in reality.  Before they went through all of that effort, they would have been wise to see if there was actually an offensive spike to explain in the first place.