Researchers say they have the first good evidence that genes play a significant role in chimp handedness, just as they are believed to in humans. They also say that--as in humans--left-handedness is often associated with developmental anomalies.
William Hopkins and colleagues at the Yerkes Regional Primate Research Center in Atlanta determined handedness by watching chimps scoop peanut butter out of a tube. They then measured similarity in hand preferences between mothers and offspring in 134 family pairs.
The researchers report in the 4 July issue of Psychological Science that birth order had a major effect on handedness in this group, as well as in 155 pairs of maternal half-siblings (siblings who share the same mother). They classified first-born chimps and those born sixth or later as being at high risk for "developmental instability"--a term for various prenatal perturbations such as hormonal irregularities associated with both first pregnancies and late ones. These conditions seemed to make left-handedness more likely, even when the mother was right-handed: The team found that among the low-risk pairs, 86% of the offspring of right-handed females were right-handed; among the high-risk ones, the proportion was only 46%.
It appears, says Hopkins, that "right-handedness is the 'norm' " and that "at least some left-handedness may be due to pathological events" around or before birth. Understanding chimp handedness "may provide insights into ... allegedly unique human psychological functions," the researchers write.
But at least one researcher, Greg Westergaard, who runs a monkey colony at LABS of Virginia in Yemassee, South Carolina, believes the findings raise the opposite question: "Given the relatively recent split between humans and apes, why are humans so much different?" he asks. Only 10% to 15% of humans are left-handed, compared with one-third of Yerkes chimps and even more of those in the wild--which suggests that there is very little specialization between the two hemispheres of the brain in chimps compared with humans.