0.10 Biology and personality  (Page 4/22)

Another important point is the issue of family studies . Usually, we think of the family as providing genetic similarity, since children inherit their genes from their parents. Second, we tend to think that families provide a common environmental situation for each of their children, particularly in small families. However, this is not always true. In the Russian literature there was a well-known case in which the first-born girl was always treated as the elder sister, even though her younger identical twin was only minutes younger. The result of the differential treatment was that the “older” girl reached most developmental milestones before her sister (Bozhovich, 1969). In a case described by the renowned Russian neuropsychologist A. R. Luria, identical twins with retarded speech had begun to develop their own autonomous language. Once separated into different classes in nursery school, however, the autonomous language disappeared (Luria, 1969). Thus, the family can have a very dramatic environmental influence, whether intentional or not, that goes against the genetic similarity due to biological relationships or even identical twinship. What then, can we conclude regarding the heritability of personality traits in humans? Certainly genetic factors play an important role, but the complexity of the human organism and its sociocultural environment makes it difficult to draw definite conclusions about exactly how much of an influence our unique genetic profile has on our individual personality. Nonetheless, psychologists have continued to pursue this important question.

Genetically Determined Dispositions

Behavior genetics is the term most commonly used to refer to studies on the influence of genetics on behavior. Most of these studies have relied on comparing identical twins to fraternal twins, other siblings, and unrelated individuals, including when possible twins who have been reared apart. These studies are often conducted in European countries that have thorough records of family histories, but one major, longitudinal study ongoing here in the United States has been the Minnesota Twin Family Study conducted at the University of Minnesota since 1983. These various sources of data, in addition to other research procedures, have helped psychologists come to some understanding of the role played by genetics in determining behavior and personality.

Psychologist Jerome Kagan is well known for his early studies on the nature of temperament . Temperament is perhaps the most salient characteristic of personality. It has been loosely described as the emotional component of our personality, as stable behavioral and emotional reactions that appear early in life and are influenced by genetic factors (Kagan, 1994). Kagan further describes temperamental categories as qualities that (1) vary among individuals, (2) are moderately stable over time and in different situations, (3) are partly determined by genetics, and (4) appear early in life. In part because they were easy to observe, the most popular temperamental qualities that have been studied are activity, irritability, and fearfulness, or as Kagan describes them: watchful inhibition vs. fearless exploration. About 10 percent of children exhibit extreme inhibition to nonthreatening, but unfamiliar, events (Kagan, 1984). Although this behavior can seemingly be altered by parental influence, subtle signs of the behavioral inhibition can be seen as the child grows, and they tend to continue into adulthood. Similarly, with uninhibited children, it is extremely unlikely that they will ever become inhibited children. Further studies on twins have helped to confirm that being inhibited or outgoing is influenced by genetics (Kagan, Kearsley,&Zelazo, 1978). Similar results have also been found with Guatemalan and Chinese children (Kagan, Kearsley,&Zelazo, 1978). As important as our emotional reactions are (see, e.g., the work of Daniel Goleman, 1995, 1998), they still do not necessarily dominate our personality: