From Wikipedia's entry: "Left-handed"

In 2007, researchers discovered LRRTM1, the first gene linked to
increased odds of being left-handed. The researchers also claim that possessing
this gene slightly raises the risk of psychotic mental illnesses such as schizophrenia.

Fortunately there's also this:

A person who is left-handed uses his or her left hand more than the right
hand. Writing is not as precise an indicator of handedness as it might seem. A
better determination of left-handedness results when the differentiation between
an individual's major motor activities versus fine or small muscle motor
activities is employed. Thus the applications of one's left-hand for eating,
writing and similar fine motor activities would be differentiated from major
motor activities such as throwing a ball or swinging a bat, etc.

And of course, then there's this:

In his book Right-Hand, Left-Hand,[27] Chris McManus of University College
London, argues that the proportion of left-handers is rising and left-handed
people as a group have historically produced an above-average quota of high
achievers. He says that left-handers' brains are structured differently in a way
that widens their range of abilities, and the genes that determine
left-handedness also govern development of the language centers of the brain.

And, then, there's this:

Famous Lefties:

Larry Bird
Bill Clinton
Kurt Cobain
Phil Collins
Matt Dillon
Benjamin Franklin
Uri Geller
Ruth Bader Ginsburg
Matt Groening
Dorothy Hamill
Mark Hamill
Joan of Arc
David Letterman
Marilyn Monroe
Martina Navratilova
Ross Perot
Joe Perry
Luke Perry
Robert Plant
Prince Charles
Keanu Reeves
Rip Torn (Rip Torn!)
Dick Van Dyke