One of the first things that anatomy students learn is that the brain is divided down the center. In most people, one half, or hemisphere, plays a dominant role. Handedness has long been a crude measure of hemispheric dominance, because each side of the brain controls the opposite side of the body. Right-handers, for instance, are likely to have dominant left hemispheres. Today researchers are realizing that studying ambidextrous children (who have no dominant hand) could yield insights into the consequences of an unusually symmetrical brain.

A team of European researchers recently assessed nearly 8,000 Finnish children and showed that mixed-handed children are at increased risk for linguistic, scholastic and attention-related difficulties. At age eight, mixed-handed kids were about twice as likely to have language and academic difficulties as their peers. By the time the children were 16, they also were twice as likely to have symptoms of ADHDand their symptoms were more severe than those of right- or left-handed students.

Ambidexterity is not causing these problems. Rather handedness is really a very crude measure of how the brain is working, says Alina Rodriguez, a clinical psychologist at Kings College London and the studys lead author. In typical brains, language is rooted in the left hemisphere, and networks that control attention are anchored in the rightbut brains without a dominant hemisphere may be working and communicating differently.

Consistent with this theory, a 2008 study by scientists at the University of California, Los Angeles, found anomalies in cross-hemisphere communication in children with ADHD. On tasks that should be the domain of the left hemispheresuch as linguistic processingchildren with ADHD seemed to be getting too much input from their right hemispheres. Rodriguez is quick to point out, however, that mixed handedness does not, by itself, indicate a malfunctioning brain and is just one risk factor among many others.

So why do some kids have overly symmetrical brains? The answer may lie in epigeneticsthe mechanism by which environmental influences affect gene expression. In 2008 Rodriguez found that women who experienced stressful life events or depression during pregnancy were more likely to give birth to children who became mixed handed, adding evidence to the idea that the experiences of a mom-to-be affect her fetuss brain development. [For more about prenatal influences on mental health, see Infected with Insanity, by Melinda Wenner; Scientific American Mind, April/May 2008.] That means that handedness, Rodriguez says, can be used with other markers to predict whos going to have problems with behavior and give parents, teachers and doctors the opportunity to intervene at the first sign of trouble.