This experiment tested how posture affects the hand preference in tufted capuchin monkeys (Cebus apella). The research indicated that in fact there may exist a preference for use of the right-hand when in an upright posture standing bipedally. This preference, according to the study is exhibited in the capuchins when reaching bipedally. However the monkeys did not necessarily exhibit a preference when using tools to probe for a treat. In addition the results showed that there was little or no bias in hand preference neither for reaching nor for tool use when standing quadrupedally. The authors link this finding with the human retreat from quadrupedalism and our subsequent right-hand preference that has been selected for almost exclusively, equally exemplified in all human cultures, through the use of complex tools.The experiment consisted of 11 male and 5 female capuchins at various developmental levels all of which contributed 50 trials each, over 8 months. In order to conduct the quadrupedal reaching analysis the researchers placed a piece of fruit on the base of the subjects cage. To evoke the bipedal reaching response the fruit was instead place 40-50 cm above the floor on the cage mesh. The probing response was evoked using an apparatus which was filled with a sweet sugar syrup the apparatus was mounted to the cage both at the bottom of the cage (quadrupedal response) and at shoulder heighth (bipedal response). In all cases a researcher took note of the hand used for the retrieval or tool use actions.In order to more accurately and systematically measure each response the researchers developed a formula that yields a handedness index (HI), a score that indicates hand preference. The formula is as follows: [(R-L)/(R+L)] , R = the number of right-handed reponses and L = the number of left-handed responses. Next this HI score was used as an absolute value so that regardless of right or left hand preference a measure is reached...
