By Poul Olson

Primatologists at Yerkes have found that, among chimpanzees, genetics and birth order strongly influence which hand individuals prefer to use for certain tasks. The study by Bill Hopkins and Jeremy Dahl, research associates at Yerkes, and research technician Dawn Pilcher will appear in the July edition of Psychological Science. It counters the prevailing view that hand preference is determined solely by environmental factors.

In their study, Hopkins and Dahl examined a total of 134 mother-infant chimpanzee pairs, divided into two groups according to the infant’s birth order. Firstborn infants and those born sixth or later to their mothers were placed in one group, categorized as the “risk group,” while infants born, second, third, fourth or fifth were placed in the other, non-risk group.

This categorization of risk is based on evidence that a mother’s first pregnancy, as well as any late pregnancies—those that occur after she already has had several offspring—tend to pose a greater risk of developmental instability for the fetus.

Among the right-handed mothers in the non-risk group, the researchers found that 86 percent of their offspring were right-handed, in contrast to only 46 percent of offspring born to right-handed mothers in the risk group.

“Once risk factors associated with birth order are taken into consideration, we find a tendency for right-handed females to give birth to offspring that develop a right-handed preference,” said Hopkins.

“On the other hand, there does not appear to be a genetic basis for left-handedness. It is atypical.”
Hopkins noted that the findings are consistent with recent studies that show significantly higher incidence of left-handedness among first- and later-born chimpanzees than among middle-born chimpanzees.

Noting the hormonal variations normally associated with first and later pregnancies, Hopkins argues that left-handedness could serve as a biological marker of a prenatal irregularity or an in utero “pathological event.”

“Our data suggest that some strong biological factors are involved in shaping the brain’s organization in a way that produces left-handedness,” he said.

Hopkins and Dahl also concluded that hand preference among chimpanzees appears to be independent of the infants’ rearing conditions.

In analyzing 155 sibling pairs, the researchers found high rates of similar hand preference, even among related chimpanzees reared apart from each other and by humans instead of their mothers.

“Hand preference appears to be highly heritable,” Dahl said. “This is evident even among genetically related individuals that have been reared apart, in different environments and by a different species.”

The researchers determined the chimpanzees’ hand preference by observing which hand they used to remove peanut butter smeared inside PVC pipe. This tube task is comparable to tasks requiring complimentary use of two hands, such as holding a nail with one hand and hammering it with the other.

Among chimpanzees, right-handedness is twice as common as left-handedness. Studies in humans have found that right-handed people outnumber lefties eight to one. Scientists have not yet explained this disparity in hand preference between humans and chimpanzees.

Although genetic differences between the two species may play a role, Hopkins suggests that the disparity could also be due to cultural and religious biases against left-handedness in some human societies.

“Some people who self-identify in these surveys as right-handed were likely born left-handed but were pressured to change at some point,” he said. “I believe left-handedness is much more prevalent in people than what is currently reported.”

Previous studies have linked hand preference to various developmental and environmental factors, including social learning and pressure, in utero fetal position, birth season, and maternal age.

In human studies, high proportions of left-handers have been found to be either mentally impaired or gifted. Being a “lefty” has been associated with greater artistic or athletic talent. On the other hand, lefties are more likely than right-handers to have reduced immune function and reproductive ability.

“What we see with left-handers are the extremes,” said Hopkins. “Being left-handed has both advantages and disadvantages.”

The two researchers plan to extend their studies of handedness to other chimpanzee populations. In addition to measuring steroid hormone concentrations of pregnant chimpanzees, they hope to conduct more extensive genetic analyses.

“We share 98.4 percent of our DNA with chimpanzees,” Hopkins said. “Our data strongly suggest that our common ancestor may have shared a gene or set of genes for handedness.”