Child Development

ences fraternal twinning rates. Maternal age can af-
fect parenting style and congenital anomalies. Some
fraternal twin parents conceive twins because they
used fertility treatments. Thus, it is important that in-
vestigators control for these differences when study-
ing related factors. When environmental similarities
are found between identical twins, it seems to be the
result of (rather than the cause of) behavioral similari-
ty. This finding suggests that environments are not
independent of genetic characteristics.

All research designs have assumptions. It is im-
portant that investigators are aware of these assump-
tions and how they limit the conclusions that can be
drawn from the study results. One way to test assump-
tions is to compare the findings of multiple studies
using different methodologies. The results of twin
studies can be verified by comparing them to findings
from family and adoption studies, for example.

Twin designs are embedded in the context of the
larger behavior genetic research paradigm. Behavior
geneticists study the genetic influences on individual
differences in behavior. The behavior genetic para-
digm begins with specifying the behavior of interest.
Whether or not biological relatives are more likely to
also express the behavior is explored next. If so, heri-
tability can be estimated from adoption and twin
studies. Relationships with the brain can then be
identified. If brain function is related to the behavior
and they share genetic influences, this suggests a spe-
cific pathway from gene function to behavior. Also,
the molecular genetic evidence can be explored.
What particular genetic variations are associated with
the behavior? Animal models can then be designed to
elucidate various components of the system. Investi-
gators are working at all of these levels of the para-
digm, and results at one level inform research at
another level. Twin studies and other behavior genet-
ic designs are powerful approaches for studying de-
velopment.

See also: GESSELL, ARNOLD; MATURATION; STAGES

OF DEVELOPMENT

Bibliography

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Goldsmith, H. Hill, Kathryn S. Lemery, Kristin A. Buss, and Joseph

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Plomin, Robert, John C. DeFries, Gerald E. McClearn, and Mi-

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Rowe, David C. The Limits of Family Influence: Genes, Experience, and

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Rutter, Michael, and Jane Redshaw. ‘‘Growing Up as a Twin: Twin-

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Rutter, Michael, Judy Silberg, and Emily Simonoff. ‘‘Whither Be-

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Kathryn S. Lemery

422 TWIN STUDIES