¶ … 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...

1292), a result that could the result of what Steinberg (2014) called sibling de-identification.
Overall, the findings of the study by McGuire and colleagues (1999) suggest that various self-concepts of competence are determined to different degrees by genetic contributions and that non-shared experience represents the primary environmental contribution. In addition, the magnitude of genetic and environmental contributions for some traits changed over time, but the traits with the strongest genetic contribution tended to be the most stable. Together these findings suggest that the genetic contribution to each trait is probably unique and complex.

Sibling De-Identification

Complicating the analysis of relative genetic and environmental contributions to adolescent behavior is sibling de-identification, which can be defined as a one sibling becoming motivated to behave differently from the other sibling (Steinberg, 2014, p. 137). The reasons for sibling differentiation could be a felt need to stand out when parents tend to treat all offspring the same. Steinberg notes that the parental practice of treating each sibling differently actually tends to minimize the risk of sibling rivalry, which can in turn minimize the felt need to de-identify.

As mention above, McGuire and colleagues (1999) discovered significant variation within sibling pairs when examining self-concept traits at both time points. Although the source of the variability was not investigated, the authors of this study called the discovered within-sibling pair differences "striking" (p. 1292). One possible source of this variation could be sibling de-identification. At the beginning of their research article, Feinberg and Hetherington (2000) propose the theory that the magnitude of similarity between siblings is directly proportional to the magnitude of de-identification (p. 1512). This type of environmental contribution, according to Feinberg and Hetherington, would go a long way toward explaining why siblings from the same family turn out so differently.

In their review of prior research findings, Feinberg and Hetherington (2000) list the various non-shared experiences that could contribute to sibling differentiation. Non-shared experiences that have been tested and found to have minimal influence on sibling differences include birth order and age difference, so researchers have become increasingly interested in the impact of differential parenting, sibling rivalry, and extra-family activities. Feinberg and Hetherington also elaborated on how complex the interaction between shared and non-shared experiences can be, with siblings reacting in the same way to distinctly different family conditions or reacting very differently to exactly the same situation. They argue, however, that quantifying the contribution from shared experiences is essential for delineating the contribution from non-shared experiences.

While substantial research has been conducted in this area of behavioral genetics, sibling de-identification has been relatively neglected (Feinberg & Hetherington, 2000). The few studies that have been conducted revealed gender and age difference determine whether sibling de-identification occurs, with siblings closer in age and of the same gender more likely to de-identify. To better understand the relative contributions from shared and non-shared experiences Feinberg and Hetherington (2000) studied the same sample of families used by McGuire and colleagues (1999). The outcome measures were antisocial, depressive, socially responsible, autonomous, and sociability behaviors, in addition to academic/cognitive competence. These measures were collected twice, three years apart.

As predicted, the results of the Feinberg and Hetherington (2000) study are complex. A smaller age difference increased the chances that a sibling would de-identify, except when the siblings are about a year apart in age. This anomaly was attributed to the strength of shared experience contributions for sibling only a year apart in age, while siblings born at the same time are influenced primarily by the need to differentiate. The author recognized that the same age sibling group contained identical twins, which would tend to skew the results due to the greater genetic contribution in the sibling pair, so they developed a correction factor to control for this influence. The authors also discovered that siblings seem to avoid the use of depressive symptoms during de-identification and instead rely heavily on pro- and anti-social behaviors; however, the latter finding is true only for siblings close in age and in early adolescence. Older adolescents who are close in age instead rely on expressions of sociability, autonomy, and self-worth to differentiate themselves from their siblings.

The study by Feinberg and Hetherington (2000) provides a good view of the various contributions to adolescent behavior. Although the magnitude of genetic contributions was not quantified, the researchers recognized the need to compensate for this contribution in the twin-filled age group. The one year age group de-identified the least, possibly because the year difference in age provided…