Posey's and Heyward's slow starts

Buster Posey has yet to play up to the standard he set in 2010, but he will. Tim Heitman/US Presswire

Last year, the National League Rookie of the Year balloting was a fight between two elite offensive rookies, Buster Posey and Jason Heyward. Combining for 7.7 wins above replacement, the young sluggers were counted on to anchor the 2011 lineups of the San Francisco Giants and Atlanta Braves, respectively. Neither has fulfilled his 2010 promise this year, with Heyward hitting .226/.329/.444 and Posey at .241/.333/.366.

One enduring bit of conventional wisdom in baseball is that players with big rookie seasons will inevitably suffer the dreaded "sophomore slump" and be disappointments after a big rookie season. But is that actually true?