Evolutionary psychologists' explanations for current behavior based on the evolution of behavior in time-distant ancestral environments are said to be ultimate explanations. They use the word ultimate in the primary sense of furthest back in time, remote, or distant. Neurobiological theories of behavior are said to be proximal explanations, in the sense that they are closely and directly linked to our current biological functioning. From a proximal neurobiological perspective, the present evidence that personality disorders have a neurobiological basis is beyond question. Debates now center on such issues as the extent of the contribution of genes to personality disordered behavior and the malleability and limits to the amelioration of personality disorders. Also of question are what particular interventions are necessary for changing personality disordered behavior and whether these interventions change across the life span.
Now let us explore the parameters of what is meant by a biological basis for a personality disorder. The major parameter for a biological basis of any behavior is genes. Genes are actually locations along chromosomes, which are structures inside the nucleus of cells. A particular gene location consists of a double strand of deoxyribonucleic acid (DNA), which is made up of only four amino acids—adenine, thymine, cytosine, and gua-nine. These amino acids accomplish two main tasks: First, their own replication (and thus, survival of the greater organism); second, the coding of larger protein molecules, which in turn control nearly all other functions of living organisms, including such basic tasks as respiration, heart rate, blood pressure, and digestion and such higher level functions as survival, intellectual endeavors, and personality traits.
Heredity patterns focus on the different forms of genes called alleles. A person's specific combination of alleles is called a genotype. The observed behavior of an individual as a result of his or her genotype is called a phenotype. There are at least three major forms of genotypic transfer: (1) major dominant gene transmission, (2) recessive gene transmission, and (3) additive gene transmission. There are also other genetic reasons for particular phenotypes that are not transmitted from parents to their children such as new mutations (that can subsequently be heritable) and changes in chromosomes (e.g., extra chromosomes or repeated DNA sequences).
One of the most well-known examples of a pathological phenotype due to a major dominant gene is Huntington's disease (HD). This progressive dementing disorder causes major dysfunction in intellectual and memory processing and eventually death. Huntington's disease is caused by a single dominant allele. These individuals inherit one dominant allele (coding for the disease) and one normal (but recessive) allele. Because the two alleles each split during reproduction, the children of HD individuals have a 50-50 chance of inheriting the dominant allele, thus 50% of the children of an HD parent will inherit HD.
The second major form of genetic transmission is due to two recessive alleles that can only express themselves in the presence of an alike, recessive, but pathological allele. If a person carries only one pathological recessive allele, this person will not have a noticeable pathological phenotype; however, by definition, they are called carriers of the disease. Reproduction with another carrier will result in a 25% likelihood that their children will exhibit the pathological phenotype, a 50% likelihood that their children will be carriers of the disease, and a 25% likelihood that their children will neither exhibit the phe-notype nor be a carrier for the disease. It is estimated that over 1,500 diseases or pathological conditions are caused by recessive alleles.
The third form of genetic transmission is less well known than the other two but accounts for millions of phenotypic traits, diseases, and conditions. This form is called additive genetic transmission, which involves multiples genes (i.e., polygenic; see DeFries, McGuffin, McClearn, & Plomin, 2000, for an overview of genetic transmission). Most complex human traits, both physical (e.g., height) and psychological (e.g., intelligence), are thought to be influenced by multiple genes. These phenotypic traits are typically measured quantitatively along some scale or dimension. Interestingly, their measurements in large groups of people often produces a normal distribution (i.e., a bell-shaped curve) with most people scoring in the middle of the dimension and fewer people toward either end of the scale. For quantitative phenotypic traits with a polygenic cause, it is not often easy to identify a point along the scale where individuals above a certain point are said to be pathological and individuals below that point are not. Nonetheless, personality disorder research conventionally measures personality disorders dimensionally yet often treats them as discrete groups or categories (Millon & Davis, 2000).
Thus, personality disorders likely have a heritable poly-genic basis, which is at least as strong as other influences. The two most traditional experimental sources for the evidence of the heritability of complex human traits such as personality disorders comes from (1) family studies where the number of affected individuals is traced in single families over generations and (2) twin studies of identical (monozygotic or MZ) and fraternal (dizygotic or DZ) twins. In twin studies, if a trait is more alike (i.e., has greater concordance) in MZ twins than in DZ twins, the trait is thought to have a genetic basis. The concordance rates are measured with correlation coefficients (r) where a coefficient of 1.00 indicates that a pair of twins (or two individuals) are absolutely the same on a trait, 0.00 indicates no relationship whatsoever between the twins, and -1.00 indicates that the twins are exactly opposite on the trait. Thus, in twin studies, a genetic influence is indicated where the MZ twin correlation is greater than the DZ twin correlation. The MZ correlation value has been used as a rough estimate of proportional influence of genes to the variability in that trait. For example, if a trait has a correlation of .62 in a group of MZ twins and a correlation of .25 in a group of DZ twins, the overall heritability of the trait can be estimated to be .62, thus 62% of the variability in the trait may be due to heritable influences. It is also important to note a common error in the interpretation of the heri-tability coefficient. A trait that is said to have a heritability of .62 does not mean that 62% of the trait is heritable. It does mean that in this polygenic trait, 62% of the variability of individual scores when measured on the trait is attributed to genetic influences.
Whereas most personality disorders have long been noted to run in families (e.g., Millon & Davis, 2000), the evidence for their heritable basis from twin studies has only recently been empirically demonstrated. The first of these was an adult twin study by Torgersen et al. (2000) who interviewed 92 MZ twin pairs and 129 DZ pairs using a structured interview for personality disorders. They found a median heritability correlation of .59 for 10 personality disorders ranging from .79 for Narcissistic Personality Disorder to .28 for Avoidant Personality Disorder.
Coolidge et al. (2001), in a study of 70 MZ twin pairs and 35 DZ twin pairs of children ranging from 5 to 17 years old, used a parent-as-respondent inventory (Coolidge Personality and Neuropsychological Inventory; Coolidge, Thede, Stewart, & Segal, 2002) designed to assess the 12 personality disorders and their features from DSM-IV-TR. They found a median heritabil-ity correlation of .75 for the 12 personality disorders with a range of .81 for Dependent Personality Disorder to .50 for the Paranoid and Passive-Aggressive Personality Disorders.
In a provocative article, Turkheimer (2000) proposed three laws of behavior genetics relating to polygenic causes. The first law is that multiple sources of evidence appear to show that all complex human behavior is heritable, at least to some extent. The second law is that the effect of genes on our behavior is usually greater than the effect of our common family influences (i.e., shared environment). The third law states that a substantial portion of the variation in complex behavioral traits is not accounted for by the effects of genes or family influences (i.e., nonshared environment). The two twin studies previously cited (Coolidge et al., 2001; Torgersen et al., 2000) appear to follow all three of Turkheimer's laws. All of the personality disorders appear to be at least somewhat heritable and some appear to be highly heritable, thus outstripping the impact of any shared or nonshared environmental influences. Typical twin study analyses include an estimate of the overall heritability of a trait, the relative differential influence of genes, the family environment in which the twins are raised (i.e., shared environment), and the unique experiences to which each twin might individually be exposed (i.e., nonshared environment). The latter statistical estimates are made using a technique known as structural equation modeling (see Neale & Cardon, 1992, for additional details). Both the Torgersen et al. and Coolidge et al. studies found that the predominant heritability model was one that included only additive genetic influences and unique or nonshared influences. Surprisingly, the effects of a family's influence on the formation of a personality disorder appeared to be the least influential.
Let's examine Turkheimer's third law with regard to these previously mentioned personality disorder studies: A substantial portion of the variation in personality disorder traits is not accounted for by the effects of genes or family influences. Genetic researchers label this factor the nonshared environmental influence, and it accounts for the reason siblings and twins are different although raised in the same family. The entire system in which a complex trait arises is characterized by a high degree of interactivity. This means that genes and environmental influences will interact with other genes and environmental influences to make simple interpretations of these separate factors almost absurd. For example, whereas a personality disorder might have nearly equally heritable and environmental contributions, particular genetic influences might predispose an individual to select a particular environment that affects the expression of that and other genetic and environmental influences.
Although all three sources of influence are involved in this interactivity, the interpretation of the nonshared environmental influences present a particularly thorny problem to psychologists, particularly in light of Turkheimer's third law. However, it does not mean that what a family teaches a child does not matter or does not affect the child's later adult personality. It may, however, indicate how siblings can be different despite being raised in the same family because the unique individual environments of each child and that child's peers may be a more potent cause of the later developmental outcomes of the child than the shared ones. However, the results of a meta-analysis of a plethora of studies designed to objectively assess these unique experiences of individuals are daunting. The meta-analysis revealed that nonshared environmental influences are exceedingly difficult to assess and quantify (e.g., Turkheimer & Waldron, 2000). Indeed, the results have been labeled the gloomy hypothesis, suggesting it may be the nonshared environmental influences that are too unique, unsystematic, capricious, and/or serendipitous to measure quantitatively— although we are likely to hear about these unique influences from our patients during treatment. Because of a high degree of interactivity between genetic and environmental influences, these unsystematic nonshared environmental influences are exposed to equally unsystematic genetic processes.
For psychologists interested in personality disorders, this may mean that additive genetic factors have the greatest influence on the formation of personality disorders. This may obviate some guilt in the family members or parents of individuals with personality disorders, but it clearly stops there. So what if a personality disorder has a genetic basis? Until highly successful biochemical genetic treatments or other biochemical interventions are developed, therapists are left to guess what the individual nonshared environmental factors might be in the formation of a particular personality disorder in an individual. Currently, there is no acceptable quantitative method for estimating these influences, leaving the assessment of these influences to the therapist's intuitions. It also leaves the necessity of these evaluations to a successful therapeutic outcome to the intuition and training experiences of the therapist. But the results of these neurobiological studies are not totally in vain. Preliminary data suggests that personality disorders are heritable (at least to some degree), but demonstrating a strong genetic basis does not invalidate or negate attempts to explain familial, environmental, and intrapsychic features that promote and maintain personality disorders. Indeed, as is true for most mental disorders, an inclusive biopsychosocial model is most appropriate for understanding the origins of the personality disorders, and, perhaps most importantly, for informing interventions.
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