Handedness is more than laterality: lessons from chimpanzees.

Is human handedness unique? That is, do our nearest living relations, chimpanzee and bonobo (Pan spp.) show species-wide handedness, as is seen in living Homo sapiens? The answer may depend on definition: Handedness (congruence across subjects and across tasks) should be distinguished from hand preference (within subject and task), manual specialization (within subject, across tasks), and task specialization (across subjects, within task). Comparison is required at both population and species level. Several methodological issues (e.g., ecological validity) are crucial, as are major confounding variables (e.g., bimanuality). The behavioral manual laterality of chimpanzees is well-studied in a variety of contexts. Especially important is tool use, which seems to enhance extent of lateralization, but this varies both within and across populations. There is much evidence for task specialization in chimpanzees, but no conclusive evidence of handedness in the strictest sense. Thus, human handedness seems to be unique among living hominoids.