In one theory that combines genetics and behavior, right-handedness
becomes dominant thanks to the value of cooperation.
Front teeth from 17 European Neandertals that lived between 130,000 and 30,000 years ago revealed scratches consistent with right-handedness
in 15 cases and left-handedness in two cases.
Frayer's research on Regourdou's teeth confirmed the individual's right-handedness
Genes for right-handedness, though, have evolved in humans alone, she proposes.
Uomini's finding of greater right-handedness on the more complex nutcracking task recalls a 1995 study led by anthropologist Linda Marchant of Miami University in Oxford, Ohio.
Marchant, who regards predominant right-handedness as an evolved trait in hominids, is intrigued by Uomini's ideas about the impact of complex tool use on hand preferences.
McGrew cautions, however, that he's "not so sure we should get too carried away with analogy to language and right-handedness
, based on one task.
Although men are slightly better represented among left-handers than women, the researchers argue that high levels of androgens have not been linked to left-handedness in boys and are actually associated with right-handedness
This, they continue, supports the contention of MacNeilage and his colleagues that, although nonhuman primates do not display the across-the-board right-handedness typical of most humans, they have a right-hand preference for fine manipulations, such as grooming.
According to her, the majority of individuals inherit a "right-shift" gene weighting the odds in favor of right-handedness and a dominant left hemisphere.
Consistent right-handedness was found in 27 subjects, and 15 showed mixed-handedness, using combinations of left- and right-hand preference for various tasks.
Everyone may start out ambidextrous, she says, but for some reason most people develop a specialization of functions in the brain hemispheres that is associated with right-handedness.