Rebecca Cook/Reuters

This generation of baseball fans has grown accustomed to records falling, both at the plate and on the mound, but this season the Chicago White Sox slugger Adam Dunn is quietly making history of a different sort. With a batting average of .161, he is on track to put up the lowest single-season batting average by any player qualified for a batting title since 1910.

The last time a qualified player had a lower average than Dunn was 102 years ago when Bill Bergen hit .139 as a starter for the 1909 Brooklyn Superbas. And at least Bergen had several excuses: he was the league’s best defender at catcher (usually considered the most important defensive position on the field) and the National League as a whole hit .244 that season, scoring only 3.7 runs a game.

Dunn, on the other hand, is often the designated hitter, rarely takes the field, and plays in a league that through Wednesday was hitting .255 with 4.3 runs a game. When taking these factors into account, Dunn’s season is all the more remarkable.

Yet in his defense, to put up such anemic numbers day after day you have to be — or have been, at least — a pretty good hitter.

Before this season Dunn was one of the most productive hitters in baseball since his debut in 2001. Going into 2011, he was riding a streak of seven seasons with at least 38 home runs, an accomplishment matched by only two other players: Babe Ruth, who also did it seven straight times from 1926 to 1932, and Rafael Palmeiro, who did it in nine consecutive seasons from 1995 to 2003.

Dunn had even overcome a low batting average before to be an above-average offensive contributor. In 2008 he hit .236 but led the major leagues in walks, had 40 home runs and added 23 doubles, giving him a deceptively good season at the plate despite the low average.

While 2008 was an extreme example of Dunn’s remaining productive in the face of a poor average, he followed that formula in all of his good seasons. His batting average is perpetually hamstrung by huge strikeout totals, but he has typically offset those disadvantages by drawing a large number of walks and maximizing his power when he does make contact.

For sabermetricians, Dunn is the prototype of the “three true outcomes” hitter, so named because walks, home runs and strikeouts are the only outcomes over which the opposing defense has no control.

Dunn is hardly alone in taking this approach to the plate; sluggers like Gene Tenace, Rob Deer, Mickey Tettleton and Dave Kingman hold a certain place in baseball fans’ imagination for their all-or-nothing approach to hitting.

As appealing as Dunn has been, though, he has shown little of his trademark clout in 2011.

Potential reasons abound, but one of the most intriguing is the fact that it coincided with a move from the National League to the American League. Learning about a new set of pitchers is always daunting, and in Dunn’s case it was made worse by the fact that, statistically, the A.L. is far superior to the N.L. Since 2005, for example, players who had at least 100 plate appearances in each league had a slugging percentage 10 points higher and an on-base-plus-slugging-percentage 17 points higher in the N.L. than in the A.L.

Whatever the reason, Dunn’s .594 O.P.S. through Wednesday represents a 308-point decline from his previous career average. Among players with 6,000 career plate appearances going into a season, the only player whose O.P.S. deviated from his career average in a season more than Dunn was Chuck Klein, who in 1940 posted an O.P.S. 313 points lower than his previous career norm.

Unfortunately for Dunn, history paints a dismal picture for players who have seasons this far below their typical rates: Klein’s career had just 114 more at-bats after 1940, Jim Bottomley was out of baseball within two years of his disastrous 1935 season, and even the great Nap Lajoie had to hang up his cleats after a brutal performance in 1916.

Working in Dunn’s favor, though, is his age. At 31, he is the youngest player to suffer such a down season after 6,000 plate appearances of good hitting.

However Dunn’s story ends, though, baseball fans can say they have been witnesses to a historically bad season by the Big Donkey in 2011.

Neil Paine is an analyst at Baseball-Reference.com, an online baseball encyclopedia of players, teams and box scores in the major and minor leagues.