0.10 Biology and personality  (Page 3/22)

Biology and personality

When talking about the role of biology in behavior, the natural starting point is the genetic makeup of each person. Our specific genetic blueprint is what distinguishes each of us as a unique individual, except for identical twins. However, since humans no longer rely on instinctive behavior, there are no aspects of personality that are specifically determined by genetics. Instead, it is more appropriate to say that our genetic makeup determines ranges within which we might develop, and our environment then determines where we fall within that range. The topics of greatest interest in the biology of personality are those topics that appear to be under a relatively greater influence of genetics than environment. But how do we determine the relative contributions of genetics and environment? Psychologists have relied mostly on twin and adoption studies.

Twin Studies, Adoption Studies, Family Studies

Twin studies have a long and interesting history in the field of psychology. Sir Francis Galton (1822-1911) studied mental abilities and is recognized as being the first to utilize twin studies. His use of identical twins, in the mid to late 1800s, is generally recognized as the first use of an experimental control group (Diamond, 1977/1997; Jensen, 1998), and the use of identical vs. fraternal twins continues to be recognized as a natural control condition by psychologists and sociobiologists (Kagan, Kearsley,&Zelazo, 1978; Wilson, 1978). Twin studies were also of interest to psychologists in the former Soviet Union (Cole&Maltzman, 1969). While Anna Freud and Melanie Klein were applying psychoanalysis to the study and treatment of children, the American physician and psychologist Arnold Gesell was comparing the achievement of fundamental developmental milestones between twins (Lomax, Kagan,&Rosencrantz, 1978), and Wayne Dennis was conducting an astonishing experiment on a pair of twins. Dennis and his wife raised the twins under conditions of minimal social and sensory stimulation. This research had a noble goal: to understand the basis for the detrimental effects of institutionalized care that was being recognized in overcrowded orphanages. However, one of the twins ended up showing signs of mental retardation (though this was attributed to an early head injury; Lomax, Kagan,&Rosencrantz, 1978). Obviously such an experiment would never be approved today, due to the ethical guidelines and oversight that have become a common part of psychological research, but twin studies done in reasonable and ethical ways continue to be an important part of psychological research.

What makes identical twins important is that they share 100 percent of their genetic material, whereas fraternal twins (like any other siblings) share an average of 50 percent of their genetic material. By extending this to families, and finally to people who have no biological relationship, we have a continuum of genetic relatedness from complete to none. This allows us to address the issue of heritability , or the degree of individual variance on some measure of behavior or personality that can be attributed to genetics. It is important to remember, however, that heritability is measured in populations (see Kagan, 1994; Sternberg, Grigorenko,&Kidd, 2005). It makes no sense to suggest, for example, that a 5-foot tall person is 54 inches tall due to genetics and then grew another 6 inches thanks to good nutrition. Adoption studies add an interesting twist to this research, since adopted children take the genetic contributions of their parents into different environmental situations, making adoption studies a useful tool for comparing the environmental contributions to the genetic contributions. However, these studies remain challenging. For example, intelligence is perhaps the most widely studied trait in terms of whether and how much it is genetic. Some of this research has been very controversial. Sir Francis Galton, who was mentioned above, believed that his research confirmed that certain races were superior to others, and that superior races had an obligation to selectively breed their best individuals for the good of future generations, as had been done (and continues to be done today) with certain breeds of dogs and horses (Galton, 1869/1997). Despite this controversial beginning to the study of genetics and intelligence, the topic has remained widely studied, but elusive nonetheless. Estimates on the heritability of intelligence range from approximately 65 to 85 percent (Gould, 1982; Jensen, 1998). However, at very early ages the genetic and environmental influences are closer to 50-50, decrease with age, and by adulthood the genetic component is almost entirely responsible for the correlation of intelligence between related individuals (Gould, 1982; Jensen, 1998). Further complicating the situation for studying children, when a wider range of extended family members are considered and cultural factors are separated from non-transmissible environmental factors, it appears that genetics, culture, and environment all play roughly equal roles (Boyd&Richerson, 1985). Indeed, culture can have profound effects on intelligence, including our definition of intelligence itself (Sternberg, 2004). Finally, returning to the controversial perspective of Galton and other proponents of the eugenics movement (the belief that superior races and classes should not mix with inferior groups), research today has demonstrated that no legitimate connection can be made between race and intelligence (Sternberg, Grigorenko,&Kidd, 2005; also see Loehlin, 1997; Williams&Ceci, 1997), and when it comes to education, IQ isn’t even the best predictor of academic performance (Duckworth&Seligman, 2005).