Human Laterality: A Multidimensional Approach* CLARE PORAC University of Victoria STANLEY COREN AND JAMES H. STEIGER University of British Columbia PAM DUNCAN University of Victoria ABSTRACT Most humans display lateral asymmetries in the use of the paired limbs and sensory organs. Four types of lateral preference (handedness, footed- ness, eyedness, and earedness) were measured in a sample of 962 individuals, ranging in age from 10 to 75 years. Factor analytic techniques were used to assess whether these aspects of laterality are manifestations of a common pro- cess or whether they should be viewed as a multi- dimensional complex. Three independent fac- tors, representing limb, eye and ear preference were obtained. This dimensional structure re- mained invariant across the separate sex and age subgroups within the sample. These results sug- gest that human laterality is a multidimensional process, and that laterality of limbs and sense organs may result from different mechanisms. The human body is bilaterally symmetrical in appearance; however, the paired limbs and sensory organs are used asymmetrical- ly. For example, in tasks where only one hand can be used, such as writing or throw- ing a ball, individuals show a consistent pre- ference for the use of one hand over the other. Foot, eye, and ear preferences are manifested in behaviours such as kicking a ball, sighting through a telescope, or press- ing an ear against a watch to hear its faint ticking. The population of humans is biased towards preferential use of the rightside in all of these coordinations (Clark, 1957; Porac & Coren, 1978). The fact that this population dextral bias seems to have been constant since the paleolithic era and appears to be independent of geographical factors (Coren & Porac, 1977; Porac & Coren, 1976) suggests a biological basis for these behaviours. Several investigators have offered genetic models (Annett, 1978; Levy, 1976) while others suggest asymmet- rical embryo or oocyte development as a factor (Corballis & Morgan, 1978; Morgan & Corballis, 1978). Such theories presume that there is a 'primary sidedness' which dictates all expressions of laterality (motor and sensory). Furthermore, it is assumed that primary sidedness is mediated by asymmetrical neural or cerebral control. There are several reasons to suspect that human laterality is not adequately de- scribed as a unitary dimension with a single origin. While physiological considerations suggest that hands and feet may share a similar control locus in a contralateral cere- bral hemisphere, such a clearly lateralized control system does not exist for the eyes and the ears. Visual information from the two eyes is available to both hemispheres since there is only partial decussation of the optic fibres at the chiasma. Similarly, audi- tory information from both ears is also bilaterally available at all stages of proces- sing beyond the superior olive. Thus, while laterality of limbs may result from the in- volvement of a speech dominant cerebral hemisphere, for example, the lateral pre- ference of eyes and ears may be due to other mechanisms which need not be corre- lated to hemispheric asymmetries. In order to ascertain empirically whether the various types of lateral preference rep- resent a unidimensional, unicausal system, one must employ techniques which directly assess the interrelationship among the va- *This research was supported by funds from the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. We would like to thank C. Behman, C. Ensworth, L. McRae, and M. Stovel for their assistance in data collection and analysis. Requests for reprints should be addressed to C. Porac. Department of Psychology, University of Victoria, Victoria, British Columbia V8W 2Y2, Canada. Canad. J. Psychol./Rev. canad. Psychol., 1980,34 (1) 91