About 10% of people are left-handed, according to expert estimates. Another 1% of the population is mixed-handed. What causes people not to favor their right hand is only partly due to genetics—even identical twins, who have 100% of the same genes, don't always share handedness.
... More important, researchers say, are environmental factors—especially stress—in the womb. Babies born to older mothers or at a lower birth weight are more likely to be lefties, for example. And mothers who were exposed to unusually high levels of stress during pregnancy are more likely to give birth to a left-handed child. A review of research, published in 2009 in the journal Neuropsychologia, estimated that about 25% of the variability in handedness is due to genetics.
Handedness is a form of human biodiversity that is only moderately heritable, which explains much about about their lack of political power as a group. Identity groups are largely constructed from relations of blood and marriage, language (e.g., signing deaf people are a strong identity group despite their problems because they have Deaf Culture, while deaf people who don't sign aren't really politically deaf), sexual relations (homosexuals), and sex.
On average there is no difference in intelligence between right-and left-handed people. But lefties do better on an element of creativity known as divergent thinking.
Six of the last 12 U.S. presidents, including Barack Obama and George H. W. Bush, have been lefties.
Left-handed people earn on average 10% lower salaries than righties, according to a recent study. Findings of some earlier studies on income have been mixed.
Despite popular misperceptions, lefties aren't more accident prone than right-handed people and don't tend to die at a younger age.
Left-handedness has been linked to increased risk of certain neurodevelopmental disorders like schizophrenia and ADHD. Mixed-handedness is even more strongly associated with ADHD.
Most people's brains have a dominant side. More symmetrical brains of mixed-handed people may explain the link to some neural disorders.
While lefties make up about 10% of the overall population, about 20% of people with schizophrenia are lefties, for example. Links between left-handedness and dyslexia, ADHD and some mood disorders have also been reported in research studies.
Recall, the standard thing nice white people always say about race -- "He just happens to be black." As George Carlin pointed out, if somebody has two black parents, "Where does the surprise part come in? I would think it would be more unusual if he just "happened to be" Scandinavian!"
But, it's much truer to say "He just happens to be lefthanded." But, because it's true, hardly anybody ever says it.