I have no idea whether this study is really legit, but it's too fascinating not to mention. Andrew Bertoli, a Ph.D. student in political science at Berkeley, has taken a look at nationalist aggression from 1958 through 2001, using this measure:
Similar to past studies  I measure aggression using the number of militarized interstate disputes (MIDs) that a state initiates. These disputes are instances where states explicitly threaten, display, or use force against other countries . This measure is commonly used in security studies, since wars happen too infrequently to be a useful measure in most statistical tests.
His conclusion? Countries that barely qualify for the World Cup are more aggressive than countries that barely miss qualification. Something about qualifying for the World Cup apparently raises a country's testosterone level and makes them more belligerent. This may or may not hold up under rigorous scrutiny, but it certainly ought to be true, don't you think?