A Norwegian study of twins expands the role of genetics in the development of a personality disorder, yet cautions that expression of a disorder depends on a combination of genetic and environmental factors.
In the study, experts posited that avoidant and dependent personality disorders are characterized by anxious or fearful traits.
People with avoidant personality disorder are often anxious in the company of others and prefer to be alone. On the other hand, people with dependent personality disorder feel more secure in the company of others and tend to need other people for decision-making and excessive support.
Prior studies have suggested that genetic factors explain about one-third of the individual differences in these personality disorder traits, while the remaining variation is best explained by environmental influences.
However, the study format used by earlier researchers was a single-occasion interview. In the new study, researchers used two different measures of assessment at two different time-points in order to better measure personality disorders traits.
In 1998, researchers coordinated testing of 8,045 young adult twins using a questionnaire that included questions about personality disorder traits. Some years later, 2794 of these twins took part in a structured diagnostic interview.
Both identical (monozygotic) and fraternal (dizygotic) twins participated. Identical twins share 100 percent of their genetic material, while fraternal twins share on average 50 percent — meaning they are genetically similar to other siblings.
Researchers then compared how similar the two types of twin pairs were on a particular trait. As such, the variation between individuals was calculated and assigned to either a genetic or environmental source.
The researchers found that two-thirds of the variation in avoidant and dependent personality disorder traits could be explained by genes and that the most important environmental influences were those unique to each twin. The environmental influences can be any factor(s) that contribute to the twins in a pair being different, e.g. the influence of different friends, teachers, activities or various life events.
Researchers state that it is important to emphasize that the term heritability does not refer to individuals per se.
Heritability is a statistic that relates to the population as a whole, and is expressed as a proportion of how much the total variation in a trait, such as personality disorders, is influenced by genes.
By using two different assessment techniques at different times, researchers were better able to estimate the role of heritability than in studies that measure personality disorder once and with one instrument only.
The dual method applied in the current study allowed researchers to capture the core of these personality disorder traits and not random effects, or effects specific to a certain time point or method of assessment, said Ph.D. student and first author of the study Line C. Gjerde.
The key finding that genes are so influential in the development of personality disorders emphasizes the importance of obtaining a thorough family history from patients with symptoms of such disorders.
However, this does not mean that personality disorders are not treatable. Gjerde emphasizes that the strong genetic influence found in the study does not imply any form of determinism or prediction of disease development. That is, if a person has a family history of personality disorders, this does not necessarily mean that he or she will develop a personality disorder.
Whether or not a genetic vulnerability leads to the expression of a certain trait or disorder depends on a complex interplay of both genetic and environmental factors.