Genetic Inheritance
In terms of the discipline of communication, the “nature or nurture” debate, as it is often called, embraces the question as to whether communicator traits and social behavior are genetically inherited or acquired through experience. The same question arises in the theoretical treatment of verbal aggression. Of course, as Zuckerman (1995) points out, “we do not inherit personality traits or even behavior mechanisms as such. What is inherited are chemical templates that produce and regulate proteins involved in the structure of nervous systems and the neurotransmitters, enzymes, and hormones that regulate them . . . we are born with different reactivity of brain structures and levels of regulators” (pp. 331–332).
Zuckerman’s (1995) observation is represented in Beatty’s (2005) mediated effects model, which specifies that genetic inheritance leads to neurobiological characteristics, which in turn leads to traits. As such, individual acts of aggression, whether physical or symbolic in nature, occur because the social stimulus excites the neurobiological systems to the degree required to implement such as response. Beatty and McCroskey (1997) argued that traits such as verbal aggression represent a person’s threshold for activation of those systems. Accordingly, a person high in trait verbal aggressiveness requires a less potent stimulus to trigger aggression than does a person low in the trait.
Behavioral geneticists have long relied on twins studies to provide indirect tests of models such as that delineated by Beatty (2005). The attraction of the twins design is that “monozygotic (MZ) twin pairs are genetically identical, but dizygotic (DZ) twin pairs share only 50 percent of their genes” (Hughes & Cutting, 1999, p. 429). Comparing the “within-pair correlations therefore provides an estimate of the proportion of trait variance attributable to genetic influences, the heritability of the trait” (Hughes & Cutting, 1999, p. 429). Once heredity coefficients are calculated, it is possible to estimate the contributions of both shared and unshared environments to the variance in the trait. (For a discussion of techniques and complicating factors, see Beatty et al., 2002.) At the outset, behavioral geneticists portioned the data into four cells: monozy-gotic twins raised together, monozygotic twins raised apart, dizygotic twins raised together, and dizygotic twins raised apart. In this way, it was possible to separate the effects of common environment from common genetic effects. However, as Zuckerman (1994) observed, “There is little difference between the corrections for identical twins who were raised apart and those who were raised together” (p. 245), which Lykken (1995) points to as the reason researchers dropped the distinction regarding whether twins are raised together or apart from formulas for calculating heritability.
Although the twins design has been described as “the perfect experiment” (Martin, Boomsma, & Machin, 1997, p. 387), the heritability coefficients estimate the direct path of genetics to traits and, therefore, constitute only indirect or suggestive evidence about the direct paths proposed in Beatty’s (2005) mediated effects model. As estimates of coefficients for direct paths, heritability coefficients represent products of intervening direct paths. Thus, with a path coefficient equal to .70 between genetic inheritance and a particular neuro-biological feature (e.g., MAO production) and a path coefficient of .70 between that neurobiological feature and trait verbal aggressiveness, the predicted correlation or heredity coefficient for trait verbal aggressiveness would be .49. Therefore, heredity coefficients greater than .50 implicate substantial coefficients for linkages not tested directly in a given study.
Eight years ago, Beatty and colleagues (2002) meta-analyzed the twins studies on aggressiveness as part of a broader meta-analytic investigation of the twins studies related to social interaction. Their literature search included an electronic search using PsychInfo, Biological Abstracts, Bioethics Online, EBSCOhost, Eric, HealthStar, and the General Science Index databases, a review of research journals that published twins studies, and a scan of the reference sections of all articles retrieved through the databases and journal searches. The twin studies of aggression that met the inclusion...