Genetic Factors

"The temperamental characteristics of a human adult are the end products of a developmental process in which original endowment, maturation, and learning have all entered in important degree."

One of the major assumptions underlying the construct of temperament is that it is constitutional-biologically based (Slabach, 1991). Genetic factors are a common denominator in literature dealing with the biological roots of temperament and correlates of temperament such as neuroanatomy, neurophysiology, and endocrinology. As Plomin (1994) suggests, it seems clear,

"...that genetics plays an important role in the etiology of individual differences in temperament. Molecular genetics provides powerful tools that can be used by behavioral scientists to identify relevant DNA differences among individuals without relying on familiar resemblance. In addition to providing indisputable evidence of the role of genetics, it will revolutionize behavioral genetics by providing measured genotypes for investigating the multivariate and longitudinal genetic issues, the links between normal and abnormal, and the interactions and corelations between genotype and environment" (p.167).

This link between genotype and the environment is supported by Eyesnk (1991) who concludes that "genetic factors cannot act directly on behavior; there must be an intervening link between genes and chromosomes on the one hand, and social behavior on the other" (p.87). Mehrabian's (1991) contention is that genetic factors determine at least 50% of temperament traits, with the balance learned in the first dozen or so years of life" (p.83). This early emergence of temperament is also supported by the research of Buss and Plomin (1986):

"Various large-scale studies of identical twins (ITs) and fraternal twins (FTs) reared together, come involving thousands of each type, have shown a fair uniformity of results. For most broad traits, such as E (Extraversion), N (Introversion), and P (Psychotocism), the estimate of heritablity range from 40% to 60% with the typical figure around 50%. Usually the correlation for ITs is about .50, while that for FTs ranges from ) to .3. For E there is some evidence of nonadditive genetic factors, as indicated by very low FT correlations. The heritability is the same for the N and P dimensions traits, but the genetic mechanism is purely additive. The results suggest little effect of shared environment; the main environmental effects seem to be specific" (p.70).

Studies of twins by researchers Torgersen and Kringlen (1973) show, "a strong genetic influence on temperament, and that future studies on this topic should not ignore the significance of the genetic influence" Thomas & Chess, 1977, p.133). Data from the study also indicates that parental attitudes and functioning, as shaped by the sex of the child or special concerns for a premature infant, at the very most have a modest etiological influence on temperament. The authors also note,

"The development process is one in which an organism from conception is emeshed in an interactive environment. From the biochemical world that interacts with the genetic material in the fertilized egg to the social and psychological world that interacts with the awakening consciousness of the newborn child, there is no separation between organis, and environment in the real world" (p.142).

Kroner (1972), in an untested study, sugests that behavioral sex differences in the neonate may result from the action of hormones in utero in sensitizing the organism's central nervous system. Calkins (1994) examined the biological-temperament link in several studies using measures of regional brain electrical activity in infants and young children. Specifically, Calkins examined the relationship between the pattern of brain electrical activity found in the frontal scalp, leads and the dimensions of temperament that were described by other resarchers as being approach-withdraw or sociability-inhibition. Calkins also notes similar research by Fox regarding brain activity as measured by EEG and hemispheric dominance.

These studies are not without their critics. Fahrenberg (1991), in reviewing the research on psychophysiological measurement of temperament, concludes that biologically oriented personality researchers,

"...by recognizing the doubtful evidence for psychophysiological traits would profit from critical reevaluation of traditional research questions and research methodology...Biologically oriented personality researchers seem to share the assumption that relatively consistent and stable traits (in particular, features of temperament) are based on the activity of anatomically distinct neuronal subsystems, particular receptor-transmitter systems, and/or on distinct properties of certain functional subsystems, e.g., differential thresholds and patterns of regulation" (p.320).

Zukerman (1991) points out that while heredibility of personality traits is much higher than most psychologists dreamed of 20 years ago, it must be emphasized that personality traits are not directly inherited, but are only manifestations of particular combinations of inherited biological traits" (p.142).