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From: rick@wicat.UUCP (Rick Moll)
Newsgroups: sci.bio,sci.med,sci.psychology
Subject: Re: The persistance of homosexuality in a gene pool
Message-ID: <476@wicat.UUCP>
Date: 2 Aug 90 01:51:26 GMT
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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.

Could you define this?  At first I thought it meant that if the
"concordance rate" were 75% that would mean that, out of all
sets of MZ twins, 3/4 would be the same (either both homo or
both hetero) and 1/4 would be different (one of each).  Under that
definition, however, there are some other things in your article
that don't make sense to me. 

>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.
...
>should prove to be as high as 80 or 90 per cent, then it
>would probably be enough to settle the nature/nurture

If 5% of the male population is homosexual, and homosexuality
is completely independent of genetics, then I get
(.95*.95) + (.05*.05) = .905  or  90.5% of pairs of male twins should
have the same sexual preference just by chance.

Using 10% homosexual, I get 82% having matching preference just by
chance.

Is the study of which you speak starting with twins where one brother
is known to be homosexual, and then checking whether the other one
is also?  In that case, 90% would indeed be startling since it would
not include the large number of uninteresting hetero-hetero pairs.
Also, are we speaking of twins raised apart here?  How was the
sample collected?

I'm not a biologist, so I appologize if everyone else in this
group knows what these numbers mean.

--Rick Moll
rick@wicat.com