The Longitudinal Twin Study (LTS) is grounded in its history and the work of the present and former IBG principal investigators, John DeFries, John Hewitt, Robert Plomin, and David Fulker, along with numerous internal and external collaborators and co-investigators. The project was launched more than 20 years ago when development funds from the University of Colorado were used to explore the possibility of working with the state Department of Health to identify and contact parents of twins. Successful cooperation with that agency led in 1983 to a 3-year award from NICHD to use the twin method to investigate the reliability of measures of infant cognition. This first LTS project set the tone for the ensuing years of the study as IBG researchers collaborated extensively with notable experts in this field, Marshall Haith, PhD and Joseph Fagan, PhD.

Two years later, the MacArthur Foundation provided its first seven years of support to LTS for testing children in their homes and the laboratory at ages 14, 20, 24, and 36 months. Later grants allowed for continued testing of the sample through age 7. The MacArthur project, was remarkable for both the breadth and depth of its measures. Renowned researchers including Robert Emde, MD, Jerome Kagan, PhD, J. Steven Reznick, PhD, and Carolyn Zahn-Waxler PhD, collaborated in the development of assessments conducted both in the families’ homes and at our laboratories measuring cognition, personality, and social functioning. During this period, IBG research associates played critical roles in ensuring the project’s success: Joanne Robinson, PhD was the Project Coordinator and Robin Corley, PhD became the Data Manager, a position which has continued to the present. The research design and many of the results from the first MacArthur grant have been published as a book: Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change, Emde, R., & Hewitt, J.K. (Eds.), Oxford University Press, 2001.

From its earliest days, some LTS assessments were designed to replicate key elements of IBG’s other major longitudinal study, the Colorado Adoption Project (CAP). Beginning at age 4, complete CAP protocols were used in addition to the ongoing MacArthur measures. From ages 9-16, CAP and LTS assessments have been identical. These assessments are currently under the direction of Sally Wadsworth, PhD. The merging of these projects has provided researchers at IBG with a unique opportunity to cross-validate the assumptions of the twin study and the adoption study while harnessing the power of each. There is no better approach to the investigation of the genetic and environmental etiology of individual differences than the longitudinal adoption design combined with the longitudinal study of adopted and nonadopted pairs of siblings and pairs of identical and fraternal twins. Separately, each design has its strengths, but also its weaknesses. Together the strengths reinforce each other and the weaknesses are minimized.

The older twins in the LTS are now participants in an NIMH funded study of executive functioning, a relatively new and rapidly growing field of research. Executive function refers to a collection of varying cognitive abilities including planning ahead and problem solving, shifting between actions easily, initiating goal-directed behavior, and regulating attention in order to complete tasks. It is believed that executive functioning is central to an individual's ability to concentrate and is tied to intelligence and self control. The study of executive function is another way to look at how individuals perceive and act in the world. By studying executive functioning we may be able to understand more about complex human behaviors, as well as find correlations between levels of functioning and certain traits or disorders.

During the session in which the executive functioning assessments are administered, the twins also participate in a NIDA funded study of “Heritable Early Indicators of Risk for Drug Dependence.” In this project, the twins were initially interviewed at age 12 and are re-interviewed at age 17. Among other important results, the findings from this study have played a significant role in contributing to our understanding of the links among different aspects of problem behavior such as conduct disorder, attention deficit hyperactivity disorder, substance experimentation, and novelty seeking by refining the concept of a latent behavioral disinhibition trait (Young, S.E., Stallings, M.C., Corley, R.P., Krauter, K.S., & Hewitt, J.K. (2000). Genetic and environmental influences on behavioral disinhibition. American Journal of Medical Genetics, 96, 684-695).

Today researchers such as Akira Miyake, PhD of the executive function study or Susan Young, PhD and Soo Rhee, PhD of the study of substance abuse turn to early measures such as the videotaped assessment of inhibition developed by Dr. Kagan to explore early antecedents of present behavior of current societal concern. In the research programs now underway, we routinely collect and analyze DNA for use in candidate gene studies such as exploring the role of the variations in the dopamine transporter genes in relation to externalizing behavior problems (Young, S.E., Smolen, A., Corley, R.P., Krauter, K., DeFries, J.C., Crowley, T.J., & Hewitt, J.K. (2002). Dopamine transporter polymorphism associated with externalizing behavior problems in children. American Journal of Human Genetics, 114, 144-149). Thus, as the project continues to move forward, we appreciate the foresight of the initial investigators who developed the LTS which has proven to be of great benefit to contemporary investigations.

Sally Ann Rhea
Project Coordinator

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