href="#fb3_img_img_f59f6119-3e79-575e-a803-84d2e699dcc2.jpg"/>
Figure 3.11 How Many People in the United States Experience Anxiety?
Source: NIH (2005), http://www.nimh.nih.gov/health/statistics/prevalence/generalized-anxiety-disorder-among-adults.shtml
Another epidemiological measure is incidence, which refers to the number of new cases of a disorder that develop during a certain period of time. Another way of thinking about incidence is to describe it as risk. That is, what is the risk of someone in a specific population developing the disorder in a given time period?
Figure 3.12 What Percentage of Inmates Have Mental Health Problems?
Source: NIH, http://www.nimh.nih.gov/health/statistics/prevalence/inmate-mental-health.shtml
Risk is also considered in a statistical manner in terms of correlation or association. For example, you can ask if there is a relationship between environmental variables such as abuse in childhood and development of a particular disorder such as depression. Subject variables such as gender are also used in this way. For example, 1 in 4 females and 1 in 10 males will have depression in their lifetime. Thus, there is a greater risk factor for females as compared with males in terms of developing depression. As with all measures of association, the presence of a relationship does not imply causation.
Research Involving Genetics
Traditional genetics studies have sought to determine which aspects of a person’s behavior can be attributed to genetic factors and which can be said to be related to environmental factors. Early research sought to find the single gene or genes involved in psychological disorders such as schizophrenia. However, with continued progress in understanding the approximately 20,000 genes of humans and the role of epigenetic factors in turning genes on and off, the picture has become much more complicated than previously thought.
incidence: the number of new cases of a disorder that develop during a certain period of time
risk: related to incidence, the likelihood that someone in a specific population will develop a particular disorder in a given time period
This has led scientists to search for endophenotypes (see Chapter 2), which lie between the genotype and the phenotype. For example, when we think about schizophrenia, we most often think in terms of the phenotypes including hearing voices and having delusions. However, endophenotypes involving cognitive and memory problems are an important part of schizophrenia (Barch & Ceaser, 2012), and are found in both individuals with schizophrenia and, often in nonclinical manifestations, in their first-degree relatives. Both cognitive and emotional-related endophenotypes are seen in other disorders.
Behavioral Genetics
One type of genetics research involving psychopathology is behavioral genetics, which is the study of genetic and environmental contributions to organisms’ behavior (see G. Carey, 2003; DiLalla, 2004; Kendler, Jaffee, & Romer, 2011; Plomin, DeFries, Knopik, & Neiderhiser, 2012, for overviews). One large question is the manner in which genes and the environment work together to shape behavior. Researchers use a variety of behavioral genetic approaches to quantify the amount of variance, which can be attributed to genetic and environmental influences.
behavioral genetics: the study of genetic and environmental contributions to organisms’ behavior
gene by environment interaction: the possibility that individuals with different genotypes may respond to the same environment in different ways
One traditional distinction has been between gene by environment interaction as opposed to gene by environment correlations (Plomin, DeFries, & Loehlin, 1977). Gene by environment interactions refer to the possibility that individuals with different genotypes may respond to the same environment in different ways. For example, some children respond to stress differently from others based on their genotype. As noted, mistreatment as a child influenced some boys differently from others later in adulthood (Caspi et al., 2002). Those boys who were mistreated in childhood and had a particular form of the MAOA gene were more likely to be violent and engage in a variety of antisocial behaviors as adults.
A gene by environment correlation concerns how certain genotypes and certain environments occur together. For example, sensation seeking as a personality trait has been shown to be inherited. Those individuals who are sensation seekers are more likely to put themselves in high-risk environments such as mountain climbing or auto racing. In this example, it is more difficult to determine the amount of variance attributed to genetic influences and environmental influences as separate factors. Thus, it is possible for genetic and environmental factors to influence each other in subtle ways.
gene by environment correlations: how certain genotypes and certain environments occur together
twin studies: a major paradigm of behavioral genetics involving examination and understanding of critical factors related to genetic influences by studying twins
monozygotic (MZ) twins: identical twins resulting from the zygote (fertilized egg) dividing during the first 2 weeks of gestation
dizygotic (DZ) twins: twins who arise from the situation in which two different eggs are fertilized by two different spermatozoa; these are called fraternal twins since their shared genes are approximately 50%—the same as that between any two siblings
adoption study: research into the phenomenon where dizygotic (DZ) and monozygotic (MZ) twins have been raised apart, providing insights into the environmental and genetic influences on human development and behavior
One major paradigm of behavioral genetics involves twin studies. Twins offer an occurrence in nature that allows for understanding critical factors related to genetic influences. This is largely based on the fact that there are two types of twins. Monozygotic (MZ) twins are identical twins resulting from the zygote (fertilized egg) dividing during the first 2 weeks of gestation. Because they both come from the same egg, their genes are identical. Dizygotic (DZ) twins, on the other hand, arise from the situation in which two different eggs are fertilized by two different spermatozoa. These are called fraternal twins, since their shared genes are approximately 50%—the same as that between any two siblings. DZ twins can be either same sex or opposite sex, whereas MZ twins must always be the same sex. By comparing the psychological traits of MZ and DZ twins, it is possible to obtain an estimate of heritability.
A classic research design is to compare the responses of MZ twins with DZ twins on particular behavioral traits such as intelligence or personality characteristics. Since it is assumed that both DZ and MZ twins would have had similar environmental influences in their family, any differences between MZ and DZ twins would be seen to be the result of genetic influences. For example, Gottesman (1991) has studied schizophrenia with this design. In these studies, a particular MZ twin was more likely (.50) to have schizophrenia if the other twin also did. In DZ twins, this was not the case.
Statistically,
