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
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.