Dear Readers

No less an organ than the british medical Journal tells us that

http://bmj.bmjjournals.com/cgi/content/full/324/7347/1195#B9

"Two well conducted and well controlled studies show that refractive
errors are much more strongly correlated in monozygotic twins than in
dizygotic twins - ref 9 , 10"

And helpfully allows a link to References to  9 and 10. (Ref 9
Provides more detail but 10 appears to be a similar study using MZ and
DZ twins)

Ref 9 shows its a 'Classical twin study' using the 'Equal environment
assumption'.

http://www.iovs.org/cgi/content/full/42/6/1232?ijkey=b10cc18d7c1f9187636c25d8f16
c93ebacbc7e2d

"Conclusions  

We have demonstrated that genetic effects are important in the
development of refractive error, with heritability of 84% to 86% for
myopia/hyperopia. The heritability of astigmatism is 50% to 65% and
predominantly involves dominant genetic effects"

And here is that environment assumption confirmed in Figure 1.

"C is correlated by a factor of 1.0 for both MZ and DZ twins (the
equal environment assumption)."

So it has to be proven that myopia is strongly genetic? Surely?   This
was all peer reviewed, right?  These guys are all trained scientists,
right?

But if we care to investigate that 'Equal Environment Assumption'

http://www.net4dem.org/cyrev/archive/Misc.%20Articles/IQ/IQandGenetics1.htm1.

For those who have only so much time to find out the facts i have
extracted 3 sections of the page.

1."Comparing monozygotic and dizygotic twins

Another area where increasing recognition of complexity is confounding
issues in IQ research is the study of twins that are raised within
their biological families. Many of the conclusions concerning
inheritance of intelligence come from comparisons between these
monozygotic and dizygotic twins. A fundamental simplifying assumption
underlying these studies is that the environment in which each member
of a twin pair develops and grows up, from conception on, is
essentially identical (Scan & Carter-Saitzman, 1979). The degree to
which twins are considered to experience the same environments is
thought to be the same for either monozygotic or dizygotic pairs.
Thus, according to the argument, the difference in concordance for any
trait between the two classes of twins can be ascribed to genetics.
This "equal environment assumption" is based on the fact that such
twins develop in the same womb and are raised together by their
biological parents a t the same time in the history o f the family an
d in the history of their society. This assumption did not encounter
serious criticism until the early 1960's (Joseph, 19 98).

One of the major problems with this assumption is that the physically
identical nature of monozygotic twins could contribute to making their
environments much more similar than that of dizygotic twins (Billings
et al., 1992). This consequence of genetic identity could result from
the ways in which both family and the larger society treat or deal
with individuals who are known to be identical and physically present
as identical. In addition, the known closer bonds between monozygotic
twins as compared to dizygotic twins can contribute to greater
behavioral similarity. Studies designed to respond to these criticisms
have yielded conflicting results. The research suffers from the same
problems of defining quantitatively those factors that could be
considered to influence behavioral development in a family and social
setting. Furthermore, contrary to the original simplifying
assumptions, the studies have revealed that there are differences in
the shared environments between the two classes of twins (Joseph,
1998). In a review of these studies, Joseph claims that, in the case
of studies of inheritance of schizophrenia, "the evidence suggests
that the classical twin method ... is [based] on the fallacy of the
equal environment assumption" (Joseph, 1998).

2.Several recent papers cast further doubt on the equal environment
assumption as it relates to early stages of development. Dr. Bernie
Devlin and co workers showed that conditions with in the womb may have
substantial effects on the concordance of subsequent scores on IQ
tests for identical twins (Devlin et al.,1997).

3."Given the examples I have provided here, Capron et al.‘s statement
that, in such studies, "... not all environmental factors may be
known..." appears a gentle understatement of the extent of the
problem"

Andrew:-)