¶ … Adolescent Behavioral Traits
Behavioral Genetics
The 'era of the genome' officially began on April 12, 2003 when the entire human DNA sequence had been declared completed (Gannet, 2008). Although there was considerable resistance to the project from the beginning, the subsequent boom in medical and genetic advances are hard to ignore. For example, BAE and colleagues (2013) recently published a genome-wide association study that searched for and found specific DNA sequences significantly associated with agreeableness and long life spans. This study would not have been possible in the pre-genome era.
Despite these remarkable advances, however, genetic research has been going on for decades in the behavioral sciences, thereby laying a foundation upon which more recent genome era discoveries can be based. To better understand this foundation, a selection of studies examining the gene-by-environment influences on child and adolescent behavior will be reviewed and discussed in this essay.
Genetic Determination of Competence
The most recent edition of Steinberg's (2014) textbook on adolescence, called Adolescence, provides a general overview of research findings on the factors influencing adolescent behavior. The overall goal of early and later studies is uncovering the extent to which behavioral traits, such as intelligence and personality, are genetically determined. A major finding is that many traits are strongly influenced by genetics, but not all (p. 135). This research has depended heavily on sibling and twin studies to better understand the magnitude of genetic and environmental contributions for a given behavioral trait or traits.
An example of one such study is that conducted by McGuire and colleagues (1999), which examined how stable selected personality traits were over time in 248 pairs of twins, full siblings, and step-siblings of the same gender. The justification for conducting this study was the mixed results of an adolescent sibling/twin study (McGuire, Neiderhiser, Reiss, Hetherington, & Plomin, 1994) and an adult twin study (McGue, Hirsch, & Lykken, 1993), which examined the heritability of self-concepts of competence. The adolescent study emphasized the importance of unique experiences for determining self-concept and downplayed the importance of shared experiences. Other traits found to be heritable included academic, athletic, and social competence, in addition to physical appearance. The adult study showed that genetics significantly influenced interpersonal, career, intellectual, family, and athletic competence, in addition to downplaying the importance of shared experiences. Findings from these and other studies contributed to the conclusion by McGuire and colleagues (1999) that shared experiences (family) play a minor and frequently non-significant role in moderating the genetic contribution to self-concepts of competence. By comparison, non-shared experiences seem to represent the dominant controlling factor.
To better understand the contributions from these different factors to adolescent self-concepts of competence, McGuire and colleagues (1999) carried out the first longitudinal study examining these traits in same-gender siblings and twins. The two questions they addressed were how stable these traits were and whether the magnitude of genetic and environmental contributions changed over time. One example of how genetic and environmental contributions can change over time was given by both Steinberg (2014) and McGuire et al. (1999), who cited research showing that the trait of intelligence becomes less determined by shared experiences over time.
The study participants enrolled by McGuire and colleagues (1999) were from intact families, with both siblings living in the same home at least some of the time. The average of the sample was 13.6-years and included 45 identical twins, 51 fraternal twins, 39 full siblings in non-divorced families, and 57 full, 29 half, and 27 unrelated siblings living in stable stepfamilies. Of the traits examined, global self-worth and morality were not significantly different between monozygotic twins and other sibling pairs, which suggest that genetics plays a minimal role in determining these traits. In contrast, the strength of correlations within monozygotic twins was significantly stronger compared to other sibling pairs for the traits of academic, social, job, and athletic competence, in addition to physical appearance, romantic appeal, and friendships. The data was then analyzed using univariate maximum-likelihood model-fitting and in general the results confirmed the correlations.
When the same traits were evaluated three years later for stability, academic, social, and athletic competence, in addition to global self-worth, were significantly stable in identical twins compared to other sibling pairs (McGuire et al., 1999). Self-concepts of friendship and global self-worth, however, experienced a greater genetic contribution three years later. In addition, there was significant variability across time for these traits in the non-twin sibling pairs, which suggests the manifestation of these traits are influenced to some extent by non-shared experiences. This finding was supported by sibling pairs...
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