A week ago Adrian told me of an article that said that twin studies showed a
low correlation of behavior. He was kind enough to give me the reference
and I now have the article.
----- Original Message -----
From: "Adrian Teo" <ateo@whitworth.edu>
Sent: Friday, March 31, 2000 3:57 PM
> Glenn,
>
> The twin studies anecdotes you provided are indeed fascinating and
> eye-catching. However, the data is alot more complicated than that, and
many
> researchers are concern about this being misinterpreted as genetic
> determinism. the concordance rates between MZ twins are typically in the
.50
> range for many personality traits, while DZ twins are in the .30 range,
> suggesting a significant genetic component (see Plomin, Chipueer, &
Loehlin,
> 1990). A correlation of .50 is actually moderately low, considering the MZ
> twins share 100% genetic material. That amounts to explaining only 25% of
> the variance in any trait. Any puzzling finding is that in adoption
studies,
> the concordance rates for biological siblings (50% in common) and adoptive
> siblings (0% in common) show very small differences only, which again
> suggest that genetic influence is small, but significant, with regard to
> complex behaviors in humans.
The article is interesting Adrian is correct that in general the
correlations are relatively low, HOWEVER, when identical twins (monozygotic-
MZ) are compared with fraternal twins (Dizygotic--DZ) the identical twins
are much more likely to behave the same. HEre is what the article says:
'The usual twin model assumes that MZ correlations can be no more than twice
the magnitude of DZ correlations, because MZ twins are only twice as similar
as DZ twins in terms of additive genetic variance. Taht is, the MZ:DZ ratio
should be less than 2. On the contrary, for some traits, the MZ:DZ ratio
substantially exceeds 2." Plomin, R., Chipuer, H.M., & Loehlin, J. C.
(1990). Behavioral genetics and personality. In L. Pervin (Ed.), Handbook of
Personality: Theory and research (pp.225-243). NY: Guilford., p. 227
"Four large twin studies of extraversion and neuroticism totaling over
23,000 pairs of twins have recently been reviewed. Table 9.1 reprints this
summary of twin correlations. For extraversion, the weighted average MZ and
DZ correlations are .51 and .18, respectively. For neuroticism, the
corresponding average MZ and DZ correlations are .48 and .20. Thus, instead
of the MZ:DZ ratio being less than 2, it is 2.8 for extraversion and 2.4 for
neuroticism. For both traits, the MZ:DZ ratio exceeded 2 for both males and
females in each of the four studies, conducted in four different countries.
Doubling the difference between the MZ and DZ correlations yields
heritability estimates of .65 for extraversion and .54 for neuroticism, but
these are clearly over estimates of heritability because they exceed the MZ
correlation itself, which is an upper-limit estimate of heritability."
Plomin, R., Chipuer, H.M., & Loehlin, J. C. (1990). Behavioral genetics and
personality. In L. Pervin (Ed.), Handbook of Personality: Theory and
research (pp.225-243). NY: Guilford., p. 227
IN another study that broke out several traits, it says,
The results in TAble 9.2 confirm the problem that MZ:DZ ratios are greater
than 2. If only the classical twin comparison of MZT and DZT is considered,
the average MZ:DZ ratio (.40/.17) is 2.4. For extraversion, the MZ:DZ ratio
(.54/.06) is 9. Similarly, MSTRA results suggest that MZ:DZ ratios are
greater than 2 on average (MZT/DZT-.52/.22=2.4), and especially for a
component of extraversion called Social Potency (MZT/DZT=.65/.08=8.1) and
for a second-order extraversion-like factor calle dPositive Emotionality
(MZT/DZT=.63/.18=3.5) In both SATSA and MSTRA, several other scales also
show this pattern of twin correlation." Plomin, R., Chipuer, H.M., &
Loehlin, J. C. (1990). Behavioral genetics and personality. In L. Pervin
(Ed.), Handbook of Personality: Theory and research (pp.225-243). NY:
Guilford., p. 231
While obviously genetics doesn't determine everything, it is a significant
portion of certain behavioral responses.
glenn
Foundation, Fall and Flood
Adam, Apes and Anthropology
http://www.flash.net/~mortongr/dmd.htm
Lots of information on creation/evolution
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