The theory was developed in October 15, 1981 by Ron Berler, a freelance journalist and Cubs fan. Berler posited in an article that "it is utterly impossible for a team with three or more ex-Cubs to win the series." Berler based this on a pattern that he observed in the post-1945 era; 1945 being the last time the Chicago Cubs made it to a World Series until 2016.
Berler cited many examples of teams with three or more ex-Cubs on their teams that reached the World Series and lost: including the 1958 Milwaukee Braves, the 1966 Los Angeles Dodgers, and the 1978 Los Angeles Dodgers. The 1978 Dodgers, according to Berler, had lost the 1977 World Series with three ex-Cubs on their roster and seemed to be doing well the next season when they traded one of those ex-Cubs (Mike Garman) away and began playing excellently. However, four weeks later, the Dodgers traded for ex-Cub Bill North and, in the words of Berler, the "team suffered an immediate tailspin and barely beat Cincinnati to the pennant." The 1978 Dodgers lost the World Series to the Yankees leading to Berler's hypothesis of three ex-Cubs making it impossible to win a championship. In addition, the 1980 Kansas City Royals lost the World Series as well with 3 ex-Cubs.
In the original article, Berler predicted that based on this pattern the 1981 New York Yankees would not win the World Series because they had five ex-Cubs on their roster (Oscar Gamble, Bobby Murcer, Dave LaRoche, Rick Reuschel, and Barry Foote), this prediction went against the odds which heavily favored the Yankees at the time. His prediction about the 1981 World Series based on this hypothesis was proved correct with the Yankees losing to the Dodgers, four games to two.
Berler relates the relationship to the inherent "cubness" that ex-Cubs take to their future teams. In the original article, he wrote that "the ballclub possesses eerie, bewitching powers over its players" and that "'Cubness'...is synonymous with the rankest sort of abject failure, and is a condition chronic among all Cubs, past and present."
Mike Royko, who popularized the term in his columns in Chicago, wrote that cubness was a "virus" where "Three or more ex-Cubs could infect an entire team with the will to lose, no matter how skillful that team might appear." Berler adopted a similar explanation in later articulations, writing that the virus "attacks all who've played for the Cubs, even if only for a single day. There is no inoculation, no cure. When traded to another team, ex-Cubs become carriers of this debilitating disease—the ticks of baseball. Any World Series team infested with three or more of them turns addled and confused, losing all ability to win."
Mike Royko developed an additional hypothesis contending that "A team with no ex-Cubs probably has the edge on a team that has even one." The key evidence provided for this was the 1986 World Series which had the New York Mets (with no ex-Cubs) defeat the Boston Red Sox (with a dramatic error by ex-Cub Bill Buckner). This hypothesis was largely discredited in the 2003 World Series with the Florida Marlins (who had ex-Cub Lenny Harris) defeating the New York Yankees (who had no ex-Cubs), though Harris did not play in the World Series.
Since 1945, of the 23 teams to reach the World Series with three or more Cubs players, only 3 have won the World Series (the 1960 Pirates, the 2001 Diamondbacks, and the 2008 Phillies). Since its articulation in 1981, the Ex-Cubs Factor has been used to predict and explain post-season and World Series defeats for many different baseball teams.
During the 1980s, the ex-Cubs factor was used to explain a number of losses by teams. It was used, in a letter to the editor, as a reason for the loss by the San Diego Padres (who ironically beat the Cubs to get to the Series) in the 1984 World Series (with three ex-Cubs) and by the collapse of the 1985 Toronto Blue Jays who had added ex-Cub Cliff Johnson on August 28 and then blew a 3-1 lead over the Royals in the 1985 American League Championship Series.
The theory became more prominent in 1990 when it was popularized by syndicated Chicago Tribune columnist Mike Royko and continued predictions by Ron Berler. On October 16, 1990, Berler again asserted the ex-Cubs factor as the reason that the favored Oakland Athletics (with ex-Cubs Scott Sanderson, Dennis Eckersley, and Ron Hassey) would lose in the 1990 World Series (which they did by getting swept by the Cincinnati Reds). It was again cited by Berler as a reason for the defeat of the league-leading 1991 Pittsburgh Pirates in the 1991 National League Championship Series to the Atlanta Braves. Mike Royko used the ex-Cubs factor to predict the playoff collapse of the 104-win 1993 Atlanta Braves (which did occur in the 1993 National League Championship Series).
The ex-Cubs factor hypothesis was used to predict the San Francisco Giants (with three ex-Cubs Shawon Dunston, Benito Santiago and Tim Worrell) loss to the Anaheim Angels in the 2002 World Series.
The ex-Cubs factor hypothesis has also been used to explain the results of the 2004 American League Championship Series (ALCS) and the 2004 World Series, both won by the Boston Red Sox. In the 2004 ALCS, the Yankees (with six ex-Cubs) lost a 3-0 game lead to the Boston Red Sox, the first time in Major League Baseball history. The Red Sox (with only two ex-Cubs) then went on to defeat the St. Louis Cardinals (with three ex-Cubs) in the World Series. The 2009 World Series resulted in a win by the New York Yankees (with two ex-Cubs) over the Philadelphia Phillies (with three ex-Cubs).
Berler's study of the ex-Cub factor since 1945 revealed only one exception since 1945: the 1960 Pittsburgh Pirates, who won the World Series with three ex-Cubs on their roster. Berler contends that one of those ex-Cubs, Don Hoak, spent so little time in Chicago, that he did not fully develop what Berler called his "Cubness". Berler quoted pitcher Jim Brosnan as saying that "Hoak is quite possibly the only man who ever conquered his Cubness".
The 2001 Arizona Diamondbacks similarly had four ex-Cubs on their roster (Miguel Batista, Mark Grace, Mike Morgan, and Luis Gonzalez) but defeated the New York Yankees in the 2001 World Series. Mark Grace declared in an post-game interview that "We beat the ex-Cub Factor!". The sympathy many fans felt for New York City following the September 11 attacks, topped off by the four ex-Cubs on the Arizona roster, seemed to stack up against the Diamondbacks. But Arizona won Game 6 in a lopsided score, and then won Game 7 in a come-from-behind finish, scoring a pair of runs in the ninth inning to win the Series. In fact, two of the four former Cubs played prominent roles in that ninth inning, Mark Grace getting a lead off single and Luis Gonzalez driving in the winning run with a single.
The 1960 Pittsburgh Pirates and the 2001 Arizona Diamondbacks (both with victories over the New York Yankees) are the only teams that have won the World Series in Game 7 walk-off victories.
The 2008 Philadelphia Phillies team was declared prior to the post-season by Berler as "doomed this year by the Ex-Cub Factor" (with three ex-Cubs Scott Eyre, Jamie Moyer, and Matt Stairs on their roster). However, the Phillies defeated the Los Angeles Dodgers (a team also with three ex-Cubs) in the National League Championship Series and the Tampa Bay Rays (one ex-Cub) in the 2008 World Series.
The Ex-Cubs Factor hypothesis has been applied to other professional baseball teams in both positive and negative references. For example, sports writer Jeff Blair has argued there exists an "ex-Expos factor" where number of former Montreal Expos players correlates with post-season success. Jim Caple for ESPN.com has similarly proposed an ex-Mariners factor (or XMF) which explores the excellent play by ex-Mariners for other teams in postseason baseball.