Journal of Comparative Psychology 1990, Vol. 104, No. 4, 309-321 Copyright 1990 by the American Psychological Association, Inc. 0735-7036/90/S00.75 Lateral Bias in Infant Chimpanzees (Pan troglodytes) Kim A. Bard Division of Reproductive Biology, Yerkes Regional Primate Research Center, Emory University William D. Hopkins Language Research Center, Georgia State University, and Yerkes Regional Primate Research Center, Emory University Carolyn L. Fort Yerkes Regional Primate Research Center, Emory University This study documents the presence, strength, and direction of lateralization in chimpanzees (Pan troglodytes) over the first 3 months of life. Nursery-reared chimpanzees (7 males and 5 females) were repeatedly assessed on a behavioral scale. Lateral bias was measured for 4 behaviors: hand- to-mouth, hand-to-hand, defensive grasp, and first step. Hand-to-mouth was significantly later- alized for the sample. Eight of the 10 chimpanzees that showed hand-to-mouth used the right hand. Lateral bias for defensive grasp was positively related to lateral bias both of first step and of hand-to-mouth. Lateral bias in hand-to-mouth was inversely related to lateral bias in hand- to-hand. Strength of lateralization increased as chimpanzees matured. These laterality effects in infant chimpanzees were expressed under conditions of emotional arousal. Moreover, degree of laterality may be a predictor of responsivity to stress. Recently, there has been a resurgence of interest in the nature and distribution of lateralized behavior among non- human primates in relation to that for humans (see Mac- Neilage, Studdert-Kennedy, & Lindblom, 1987). There is considerable debate over the factors that affect the distribution of lateralized behavior in both human and nonhuman pri- mates. Therefore, it is important to discern those lateralization traits that human and nonhuman primates may share. Approximately 90% of humans are considered predomi- nantly right-handed (Annett, 1985), although the notion of clear dichotomies of right- and left-handers is considered to be outdated (Healey, Liederman, & Geschwind, 1986). In- This study was supported by National Institutes of Health (NIH) Grant RR-00165 to die Yerkes Center, NIH Grant RR-03591 to R. B. Swenson at the Yerkes Center, and by National Institute of Child Health and Human Development (NICHD) Intramural Research Program Funds through the Laboratory of Comparative Ethology, NIH. Additional support was provided by NICHD Grant 06016 to the Yerkes Regional Primate Research Center and National Research Service Award Research Training Fellowship HD-07105 to Kim A. Bard. We gratefully acknowledge the assistance provided by the Divisions of Reproductive Biology and Veterinary Medicine, the animal care staff in the Great Ape Nursery, and numerous student assistants at the Yerkes Research Center. Grateful appreciation is extended to two anonymous reviewers and Jeannette Ward for their very constructive critiques. We are indebted to Stephen J. Suomi for his support and encouragement, Josh A. Schneider for photographic services, and Kathleen A. Platzman, a Brazelton-certified examiner, for training and for comments on the manuscript. Yerkes Regional Primate Research Center is fully accredited by the American Association for Accreditation of Laboratory Animal Care. Correspondence concerning this article should be addressed to Kim A. Bard, Division of Reproductive Biology, Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322. stead, the distribution of hand preference is considered to form a continuumfrom strongly right-handed to strongly left- handed persons. Additionally, hand preference seems to be influenced by factors such as heredity (Annett, 1985; Levy & Nagylaki, 1972), neurological insult (Geschwind & Gala- burda, 1985), development (Young, Segalowitz, Corter, & Trehub, 1983), and task demands (Fagot & Vauclair, 1988; Healey et al., 1986; Hopkins, Washburn, & Rumbaugh, 1989). For example, a greater incidence of left-handedness has been linked to mental retardation and reading disabilities as well as other developmental disorders (Pipe, 1988; Satz, Soper, & Orsini, 1988). Given the influence of these factors, many argue that bio- logical components or dispositions may influence both hand preference and other lateralized processes in humans (Witel- son, 1987). This argument is bolstered by the fact that ana- tomical asymmetries exist in the brain of both human adults and neonates (Geschwind & Levitsky, 1968; Wada, Clarke, & Hamm, 1975; Witelson, 1977; Witelson & Pallie, 1973). Further support is found in the observation that other asym- metries are lateralized very early in life, such as turning responses (see Turkewitz, 1977) and processing of speech and nonspeech sounds (Molfese & Molfese, 1979). A growing literature suggests that nonhuman primates evi- dence anatomical asymmetries similar to those observed in humans (Cain & Wada, 1979; Falk, 1978; Falk, Cheverud, Vannier, & Conroy, 1986; Holloway & de la Costelareymon- die, 1982; LeMay, 1985; LeMay & Geschwind, 1975; Yeni- Komshian & Benson, 1976). Moreover, some behavioral stud- ies have revealed lateralized processing for species-specific vocalizations (Heflher & Heffner, 1984; Petersen, Beecher, Zoloth, Moody, & Stebbins, 1978), tones (Dewson, 1977; Pohl, 1983), facial discrimination (Hamilton & Vermeire, 1988; Morris, Hopkins, Bolser-Gilmore, & Washburn, in press), line orientation (Hamilton, 1983; Hopkins & Morris, 309