Children born early are almost twice as likely to be left-handed rather than right-handed, according to author Sharon Moalem, MD, PhD. Moalem explores the health implications of handedness in her new book, Inheritance (2014), and she presents some theories as to why a person favors one hand over the other.

There are a lot of silly names associated with being left-handed: southpaw, mollydooker, corky dobber, sometimes even goofy-footed. Medical scientists often refer to southpaws as sinister but that's not to imply they are evil or shady in any way; sinister is merely the Latin word for left and many medical terms have Latin origins.

One theory that attempts to explain the connection between preemies and left-handedness is exposure in the womb to some element that triggers gene expression for both left-hand dominance and early delivery. No one has identified the element that influences genetic expression this way nor have any genes governing handedness been discovered.

The genetic theory remains in question partly because identical twins don't always share the same hand dominance although they share identical DNA. One may be right-handed but the other left-handed. The reason remains unknown but one geneticist, according to Moalem, says the fact that identical twins may differ only proves his case for a genetic explanation.

Amar Klar is a senior investigator at the National Cancer Institute's Gene Regulation and Chromosome Biology Laboratory and he's been working on the genetics of handedness for many years. Klar's theory is that when one identical twin is left-handed, it's a sign that the twins inherited the recessive left-handed gene(s) from both parents. Klar speculates that when the handedness gene from each parent is recessive (left-handed), each child has a 50-50 chance of going right or left. But, still, handedness genes have never been identified.

Even without genetic understanding of handedness, there are certain medical conditions more likely to affect left-handers than right-handers:

  • Attention deficit hyperactivity disorder (ADHD)
  • Certain mood disorders
  • Dyslexia
  • Premenopausal breast cancer

A Danish study found that left-handed 8-year-olds with ADHD are more likely to still be experiencing symptoms of ADHD at age 16 than right-handers diagnosed with ADHD at age 8.

Goofy-footed? Thank Walt Disney for that one. Regardless of a person's dominant hand, the dominant foot is likely on the same side. Surfers who are left-foot dominant (left foot at the rear of the surfboard) are said to be goofy-footed because a 1937 animated short film by Disney (Hawaiian Holiday) depicted Goofy trying to surf in that stance: right foot forward, left foot back.

Source: Moalem, Sharon. "Picking Sides: How Genes Help Us Decide Between Left and Right (excerpt)." The Crux / Discover Magazine. Kalmbach Publishing Co. Apr 15, 2014. Web. Apr 22, 2014.