Birth order and hand preference in chimpanzees (Pan troglodytes): implications for pathological models of handedness in humans.

Approximately 85% to 90% of humans report themselves as being right-handed (Annett, 1985; Porac & Coren, 1981). Explaining the pervasive degree of right-handedness in the human species has fostered considerable theoretical debate and empirical investigation. Genetic, biological, and environmental models have been proposed to explain this population-level right-handedness, each with varying amounts of empirical support (Annett, 1999; Collins, 1985; Corballis, 1997; Laland, Kumm, Van Horn, & Feldman, 1995; Previc, 1991; Provins, 1997; Trevarthen, 1996). Equally important are theoretical models proposed to explain the occurrences of left-handedness or non-right-handedness in human populations. That genetic factors may play a role is supported by the observation that approximately 70% of individuals born to two left-handed parents are right-handed, which is significantly lower than the proportion of fight-handed individuals born to two right-handed parents (McGee & Cozad, 1980; McManus & Bryden, 1992). Genetic explanations account for only part of the variance, however, leaving error or other factors to explain the remaining variability. Rather than emphasize genetic factors, others have suggested that both pre- and postnatal environmental factors play a role in determining non-right-handedness. Such factors include but are not limited to birth season, birth order, maternal age, and prenatal hormones. In these models, right-handedness is perceived as being the normative path of development, and therefore non-right-handedness reflects a deviation from normal development that is due to pathological factors (see Satz, Soper, & Orsini, 1988). Of specific interest to this study are the reports focusing on the effect of birth order on hand preference in humans. Bakan and colleagues (Bakan, 1971; Bakan, Dibb, & Reed, 1973) first reported and theorized about the relationship between birth order and hand preference in humans. Bakan (1971) reported that there was a higher percentage of left-handedness in first- and latter-born offspring (defined as more than 4 birth orders) and postulated that the effects were due to birth trauma experienced by offspring born to primiparous or older women. Bakan (1971) proposed that birth trauma is more likely in women who are giving birth for the first time or for older women who present a higher risk group for prenatal insult. Researchers who have attempted to replicate Bakan's early reports have come up with equivocal results, with most of the discrepancies between studies centering on the issue of sampling procedures (Annett & Ockwell, 1980; Badian, 1983; Bakan, 1977, 1978; Coren & Porac, 1980; Dellatolas, Curt, & Lellouch, 1991; Hicks, Evans, & Pellegrini, 1978; McKeever, Suter, & Rich, 1995; Nachshon & Denno, 1986; Perelle & Ehrman, 1994; Schwartz, 1977; Searleman, Porac, & Coren, 1989; Tan & Nettleton, 1980). Specifically, there are significant relationships between socioeconomic status (SES), rate of pregnancy, and birth complications that are likely due to factors associated with maternal lifestyle (such as nutritional habits, medical care, etc.). For example, less well-off individuals typically have more offspring, poorer prenatal care, poorer nutritional habits, and more birth complications compared with well-off individuals (Birch & Gussow, 1970). According to Bakan (1977), it is critical to have equal representation of subjects from various SES backgrounds to control for these extraneous variables, which he argues has not been the case in studies in which his findings were not replicated (e.g., Hubbard, 1971; Schwartz, 1977). The purpose of this study was to examine the effect of birth order on hand preference in a sample of captive chimpanzees. Theoretically, the effect of birth order on hand preference in captive chimpanzees (as well as other primates) is of interest because the hypothesized SES confounding variables debated in the literature on humans would not be potential confounds in a sample of captive chimpanzees (or other primates) where the environment and access to resources are more uniform. In other words, the methodological issue of whether there was adequate sampling of subjects to control for SES variables would never be considered in a sample of nonhuman primates. Presumably, any significant effect of birth order on hand preference would be solely due to biological factors associated with parity or maternal age or both. Pragmatically, chimpanzees offer a good model for testing the effect of birth order on hand preference because they are biologically and genetically very similar to humans, but there is no socioeconomic variation in a captive colony. Additionally, chimpanzees have been reported to exhibit population-level right-handedness, and therefore non-right-handedness can be viewed as reflecting deviation from a species-typical norm (Hopkins, 1996; but see McGrew & Marchant, 1997). Finally, chimpanzees have a relatively long gestation period (approximately 230 days), exhibit stages of prenatal development comparable to humans, display periparturitional behaviors that resemble many of those observed in humans (Nissen & Yerkes, 1943), and exhibit a wide range of similar puerperal pathologies (Dahl, 1999).