On 9 Jan 2003, Jean-Marc Perreault wrote:
> Teaching my intro class today, a good question came
> up: It has been found that Identical twins reared
> together are more similar personality wise to one
> another than identical twins reared apart. But are
> identical twins who were raised apart more similar
> than fraternal twins reared together?
>
> I suspect that the answer is yes, but would like
> confirmation of this.
Interesting question, but the premise is wrong. Bouchard et al
(1990), of the famous (or infamous, depending on your politics)
Minnesota Study of Twins Reared Apart, reported (MZA = identicals
reared apart; MZT= reared together)
MZA MZT
Mean of 11 Multidimensional
Personality Scales 0.50 0.49
Mean of 18 California Psychol
Inventory 0.48 0.49
So he found no difference in identicals in personality whether reared
together or apart. That's pretty startling when you think about it.
It means that you would grow up to be you whether you were raised in
your own family or in someone else's. Shared family influence on
personality = zero.
I tried to see how well this result held up in more recent work.
Plomin and Caspi (1999) discuss the issue, and while they don't give
figures for MZA and MZT, they summarize various data by saying that
"for self-report personality questionnaires...shared environment
[accounts for] 0%". This implies, once again, no difference between
MZAs and MZTs.
What about for peer ratings rather than self-ratings of personality?
Plomin and Caspi present estimates in their Figure 9.1 for the Big
Five personality traits. They show (my eyeballing) shared environment
effect of about 20% for extraversion, 20% for agreeableness, about 5%
for openness to experience, none for neuroticism, and none for
conscientiousness. So for the first two factors, there would be some
modest increase in similarity for identicals together, but not for
the other three factors.
As for comparing MZA with fraternals together, I'm not sure why Marc
would want to do this. MZs reared apart have the same genes but
different environments; fraternals together have different genes but
similar environments. So differences between the MZs apart and the
fraternals together could be either due to changing genetic
similarity or changing environmental similarity or both. That
wouldn't be informative.
But I tried to see if I had any data anyway. The Minnesota study
folks (Tellegen et al, 1988) had some in an earlier paper. They
reported correlations for 14 scales of the Multidimensional
Personality Questionnaire for MZA, MZT, DZT (fraternals together),
plus DZA, which I'll ignore. (Actually, this was probably the same
data or a subset of what went into the 1990 paper I quoted above. The
three extra scales were "higher-order ones")
For every scale, MZAs showed higher correlations than for DZTs, just
as Marc predicted. I calculated the average for all 14 scales: for
MZA it was .49, for DZT it was .23 But once again, there was
little difference for MZTs (.54) from MZAs.
Stephen
Bouchard, T. et al (1990). Sources of human psychological
differences: the Minnesota Study of Twins Reared
apart. _Science_, 250, 223--
Plomin, R., amd Caspi, A. (1999). Behavioral genetics and
personality. In: Pervin, L. ed. Handbook of Personality.
Tellegen, A. et al (1988). Personality similiarity in twins reared
apart and together. Journal of Personality and Social Psychology
54, 1031--
______________________________________________________________
Stephen L. Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
Bishop's University e-mail: [EMAIL PROTECTED]
Lennoxville, QC J1M 1Z7
Canada
Dept web page at http://www.ubishops.ca/ccc/div/soc/psy
TIPS discussion list for psychology teachers at
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