P1: GDP/GVG/GFQ P2: GVG
Psychiatric Quarterly [psaq] ph071-psaq-346687 November 7, 2001 8:34 Style ﬁle version Nov. 19th, 1999
Psychiatric Quarterly, Vol. 73, No. 1, Spring 2002 (
TWIN STUDIES IN PSYCHIATRY AND
PSYCHOLOGY: SCIENCE OR
Jay Joseph, Psy.D.
Twin studies are frequently cited in support of the inﬂuence of genetic factors
for a wide range of psychiatric conditions and psychological trait differences.
The most common method, known as the classical twin method, compares the
concordance rates or correlations of reared-together identical (MZ) vs. reared-
together same-sex fraternal (DZ) twins. However, drawing genetic inferences
from MZ–DZ comparisons is problematic due to methodological problems and
questionable assumptions. It is argued that the main theoretical assumption of
the twin method—known as the “equal environment assumption”—is not ten-
able. The twin method is therefore of doubtful value as an indicator of genetic
inﬂuences. Studies of reared-apart twins are discussed, and it is noted that
these studies are also vulnerable to methodological problems and environmen-
tal confounds. It is concluded that there is little reason to believe that twin
studies provide evidence in favor of genetic inﬂuences on psychiatric disorders
and human behavioral differences.
KEY WORDS: twins; twin method; genetics; psychiatry; psychiatric genetics; schizo-
Jay Joseph, Psy.D., is afﬁliated with La Familia Counseling Service, Hayward,
Address correspondence and requests for reprints to Jay Joseph, P.O. Box 5653,
Berkeley, CA 94705-5653; e-mail: email@example.com.
2002 Human Sciences Press, Inc.