Research With Twins
Introduction to the Twin Method in Research
Twins occur in about 1 in 85 human births. Twins come in two types: fraternal or dizygotic (DZ) and identical or monozygotic (MZ). The difference between the two types of twins stems from a difference in how they began life – from the fertilization of two separate eggs (resulting in DZ twins) vs. from the fertilization of a single egg that later split in two (resulting in MZ twins). This difference in fertilization results in one type of twins – DZ – sharing an average of 50% of their genetic material (much like non-twin full siblings) and the other type of twins – MZ – sharing 100% of their genetic material. This difference in amount of shared genetic material sets the stage for a marvelous natural experiment and it is the basis of the twin method for research.
The twin method is used
to quantify the magnitude of the contribution of genetic and environmental
factors to individual differences in a trait or behavior. The twin method
of research entails the collection of data on both kinds of twin pairs
(MZ and DZ). Data from MZ pairs can then be statistically compared to
the data from DZ pairs in order to assess the relative contributions of
genetic and environmental factors to the trait or behavior of interest.
Given that MZ twins share twice the amount of genetic material as DZ twins,
then a greater similarity found among MZ twins as compared to DZ twins
would be indicative of genetic influence. However, if DZ twins were as
similar on a trait or behavior as MZ twins, then shared environmental
influences (those factors shared by members of a family that make them
more alike) would be indicated. Finally, if MZ and DZ twins did not look
similar on a trait or behavior, then non-shared environmental influences
(those factors that are unique to members of a family which serve to make
them different from each other) would be indicated. These types of conclusions
can be drawn from twin data if we make some important assumptions. Namely,
that MZ and DZ twins have equal environments (i.e., MZ twins are not treated
more similarly in the environment than DZ twins) and that twins are representative
of the population (i.e., there is nothing unusual about twins per se that
would make their data on a trait or behavior markedly different from data
Twin Research in the Taylor Lab
Florida State Twin Registry
The Florida State Twin Registry was established in 2002-03 by Dr. Taylor. Twins pairs aged 18 and older were recruited on the main campus of Florida State University and from the surrounding Tallahassee area for an initial research study. The study was aimed at examining genetic and environmental factors associated with the co-variation of personality features, cognitive functioning, and psychological functioning. Approximately 200 adult twin pairs joined the Registry and 120 of those completed the initial study.
Since then, the Registry has grown through collaboration with researchers in the Florida Center for Reading Research. A multidisciplinary group of researchers led by Dr. Richard Wagner was awarded a Center Grant from the National Institute of Child Health and Development to study reading disability. Dr. Taylor teamed up with Dr. Chris Schatschneider on one of the Center Grant projects, which has allowed Dr. Taylor to add over 2,100 pairs of twins in Kindergarten through Fifth Grade to the Registry. The Registry continues to grow through that grant-funded project and will serve as a resource for future twin research in the Taylor Lab.
Additional twin research is conducted using archival data from the Minnesota Twin Family Study (MTFS; PIs: William G. Iacono and Matt McGue), an ongoing longitudinal study of substance use and related behaviors in twins and their parents. The MTFS is conducted at the University of Minnesota in Minneapolis, MN. Briefly, the MTFS includes data from same-sex, reared together twins in two age cohorts: 11 and 17. Data are collected on the twin pair and their biological parents during a day-long assessment that includes clinical interviews, self-report on personality and other psychological and environmental factors, and a psychophysiological test battery. Dr. Taylor's research using the MTFS data has focused largely on genetic and environmental influences on antisocial behavior. For more information on the MTFS, click here to visit their website.