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From: dhinds@portia.Stanford.EDU (David Hinds)
Newsgroups: sci.bio,sci.med,sci.psychology
Subject: Re: The persistance of homosexuality in a gene pool
Message-ID: <1990Jul31.051219.28656@portia.Stanford.EDU>
Date: 31 Jul 90 05:12:19 GMT
References: <10615@cs.utexas.edu> <1990Jul29.050038.24791@wolves.uucp> <32214@cup.portal.com>
Organization: AIR, Stanford University
Lines: 58

In article <32214@cup.portal.com> geoffp@cup.portal.com (Geoffrey Scott Puterbaugh) writes:
>
>     The concordance rate for homosexuality among male MZ (identical)
>twins is the interesting number addressed by these studies.
>If the theory of environmental causation were true, then one
>would expect no concordance for homosexuality at all; that
>is, homosexuals would appear at the basic Kinsey rate of 5
>to 10 per cent of the male population.
>
>     But that is not the number which medical research gives
>us.  If we add up all the pairs reported in these eight
>articles (including those summarized in Zuger and in Heston
>and Shields), we obtain the following totals: 65 pairs of
>male MZ twins, 50 of whom are concordant and 15 discordant.
>This amounts to 77 per cent concordance among identical male
>twins.  (Out of caution, this count excludes all females and
>all cases where schizophrenia was present.)
>
>     The true concordance rate is not yet known.  If it
>should prove to be as high as 80 or 90 per cent, then it
>would probably be enough to settle the nature/nurture
>debate.  This conclusion may seem startling, since many
>people believe that even one discordant MZ pair must
>disprove the entire genetic case.  Identical twins have
>identical genes: if they are discordant in regard to
>homosexuality, then clearly something else is at work.
>
>     We must, given these figures for MZ twin pairs, compare
>them to the figures for DZ (fraternal) twins.  The DZ twin pairs
>show no concordance at all for homosexuality, or a very slight
>concordance.  In every case so far known to medical
>research, the DZ concordance rate has been so much lower
>than the MZ concordance rate that the DZ rate has generally
>been compared to the rate for siblings.  And the DZ rate is
>consistent, whether the DZ twins were raised together or
>apart.

    Is the MZ concordance rate consistent, whether raised together or
apart?  I doubt that there is enough data to tell.  Sampling biases
also seem to be frequent problems in twin studies of all kinds, and I
would view all small twin studies with caution.  The standard argument
against explaining MZ concordances as evidence of genetic linkage is
that MZ twins are thought to have much more similar environments than
DZ twins.
    If DZ twins and siblings really do show almost no concordance, then
the data would imply that sexual preference is controlled by sufficiently
many genes that a sibling's 50% genetic identity has no predictive power
for the trait.  It would also indicate the absence of any significant
environmental influences.  While the absence of any major gene effects
tends to be the rule for complex human characteristics, this is usually
accompanied by substantial environmental effects.  All genetic character-
istics are modulated by environmental influences to some degree; it would
be strange for such an apparently complex trait (as judged by MZ vs DZ,
taken at face value) to show no environmental influence (as judged by DZ
vs. population average).

 -David Hinds
  dhinds@popserver.stanford.edu