Three generations of identical female Russian twins. There is some evidence that identical twins are more likely to be dressed alike; there is no
evidence, however, that this increased similarity is linked to the types of behaviors that psychologists typically study, such as mental abilities or
psychopathology. (Gerald Davis/Phototake NYC)
environmental factors on behavior and development
is specific to each child, depending on individual
characteristics, such as personality. Thus, parenting
is influenced by child characteristics as well as paren-
tal characteristics.
Using simultaneous equations model fitting, the
classic twin design has been expanded to the multi-
variate case to examine whether or not the same ge-
netic and environmental factors influence two or
more behaviors. This approach is useful for studying
etiology, in which a researcher investigates why two
behaviors—such as depression and anxiety—are like-
ly to occur together. Using the same example, results
from twin studies in childhood (conducted by Anita
Thapar and Peter McGuffin) and adulthood (con-
ducted by Ken S. Kendler and colleagues) suggest
that a shared genetic factor primarily accounts for the
co-occurrence of depression and anxiety. In addition,
specific measures of genes (e.g., variations of the dop-
amine receptor gene) and environments (e.g., socio-
economic status) can be incorporated into the model
to determine their individual effect. And assumptions
of the twin method can be systematically tested.
There are two main assumptions of the twin de-
sign. The first is that twins are representative of the
general population. Are twins and single-born chil-
dren comparable? Because obstetrical complications
and congenital anomalies are more common in twins,
behaviors that are confounded with these complica-
tions are more common in twins. These differences
might be important for some characteristics. In fact,
twins are overrepresented in cerebral palsy and cog-
nitive delays. On the other hand, twins are compara-
ble on temperament and personality measures, and
the proportion of twins in psychiatric clinics is no
greater than the proportion for the population as a
whole.
The second main assumption of the twins-reared-
together design is equality of environments. Are iden-
tical and fraternal twins equally influenced by envi-
ronments that are important for the behavior under
study? If this assumption is violated, then the in-
creased similarity of identical twins over fraternal
twins may be partially due to environmental, rather
than genetic, factors. For example, there are greater
discrepancies in birthweight between identical twins
than fraternal twins. Also, high maternal age influ-TWIN STUDIES 421