One of the most common methods used to examine the differential influence of nature and nurture
are twin studies. This methodology involves comparing the behaviour of monozygotic or identical (MZ)
twins who share identical genetic makeup to dizygotic (DZ) or fraternal twins who share only 50 percent of
their genetic makeup (Steen, 1996; Rowe, 1990). The underlying assumption of this design is that MZ and
DZ twins share essentially the same environment and so any difference between twin-type can be attributed
to genetic or inherited influences (Blackburn, 1993; Steen, 1996; Rowe, 1990). Accordingly, it is expected
that the extent to which MZ twins are similar (concordance) will exceed that of DZ twins (Blackburn, 1993;
Hollin, 1992; Raine, 1993). Indeed, numerous twin studies and reviews report that concordance for MZ
twins exceeds that of DZ twins in relation to criminal behaviour (e.g., Christiansen, 1977; Cloninger &
Gottesman, 1987; Ellis, 1982; Rowe, 1986; Rowe & Osgood, 1984), with Raine (1993) reporting an average
concordance of 51.5 percent for MZ twins compared to 20.6 percent for DZ twins. Therefore, these studies
lead to the conclusion that genetics exert a substantial influence on criminality.
In attempts to control for environmental factors being confounded (a common criticism of the
abovementioned twin study method), researchers have used an even more robust methodology whereby they……
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