Bill James states that there is a bias favoring players from earlier eras because there was greater variance in skill levels at the time, so "the best players were further from the average than they are now". That is, in modern baseball, it is more difficult for a player to exceed the abilities of his peers than it was in the 1800s and the dead-ball and live-ball eras of the 1900s. James's criticism originates from the evolutionary biologist Stephen Jay Gould who, in 1996, published the book Full House which argued the same point with respect to batting averages. The bias mentioned by Gould and James was confirmed in a statistical study which showed that ranking lists based on WAR do in fact include too many players from the earlier eras. This study challenges the stance that WAR properly adjusts for era differences.
