In Brief

What makes a chimpanzee left-handed--nature or environmental influences? Mostly nature, according to a recent study of chimp mothers and infants.

That result refutes past animal research, which suggested that behavioral reinforcement and other external factors largely determine animals' hand--or paw--preferences. In contrast, this study from Emory University's Yerkes Regional Primate Research Center found that a chimp's preference for right or left stems mainly from birth order and genes.

To reach that conclusion, researchers Bill Hopkins, PhD, and Jeremy Dahl, PhD, investigated hand preference in a group of 134 mother-infant pairs and a group of 155 sibling pairs. In the first sample, the researchers placed first-born and sixth-born infants in one group at "high risk" for being left-handed. They placed the rest of the infants in a "low risk" group and determined the handedness of babies in both groups.

They theorized that the early- and late-born babies were more likely to favor their left hands due to hormonal fluctuations, birth trauma and other developmental instability associated with first and later pregnancies.

As expected, they found that more of the babies of right-handed mothers--86 percent--were right-handed in the low-risk group, while slightly more of the babies of right-handed mothers--54 percent--were left-handed in the high-risk group

"We found that left-handedness is atypical in chimps," says Hopkins. "And the fact that chimpanzees' socioeconomic status is identical indicates that the explanation for handedness is biological."

To further probe for a biological basis of handedness, the researchers examined handedness among pairs of chimp siblings. That study also revealed that--independent of being first- or sixth-born and regardless of being reared together or apart--siblings tended to favor the same hand as one another, indicating "the genetics of their handedness is very strong," says Hopkins.

What might these findings suggest about humans? Probably that biology plays a major role in human handedness too, but that culture has mediated that, says Hopkins. Whereas more than a third of chimpanzees are left-handed, only about 10 to 20 percent of people are, likely because right-handedness is taught and valued in human society, he says.

"The point is that what we see in chimps is truly biological," says Hopkins, whose findings are published in the current issue of Psychological Science (Vol. 12, No. 4). "What we see in humans is biology plus culture."

--B. MURRAY