The "Mars Effect" is the name given to Michel Gauquelin's "astrobiological" claim that great athletes are born in numbers indicative of a non-chance correlation when Mars is in certain sectors of the sky. If this were so, who knows what it would demonstrate, but astrologers love this kind of "scientific" support for their theories that the things in the sky are more important than the things in your life on earth in determining who and what you become.
In any case, what Gauqelin claims about Mars and athletes isn't so, according to a study by seven French scientists. They took a sample of 1,066 French athletes and compared them to 85,280 others for birth times, dates and location of Mars at birth. The study didn't support the "Mars Effect."
Gauquelin preferred to call his work in this area "astrobiology" rather than astrology. He also claimed to find a signficant correlation between Jupiter and military prowess, as well as between Venus and artists. Stay tuned for updates on this exciting new science!
See related entry on astrology.
further reading
Dutch Investigations of the Gauquelian Mars Effect by J. W. Nienhuys
The True Disbelievers Read about the exciting history of this subject, complete with charges of disingenuous skepticism by Marcello Truzzi and Jim Lippard - a real potboiler!
Benski, Claude, et. al., The "Mars Effect" (Amherst, NY: Prometheus Books, 1996).