This forum item has a $5,000 answer (and a zero dollar answer).
http://psych.colorado.edu/~carey/Bouchard/BouchardPrize.php
Greg Carey is offering an economic stimulus package of $5000 cash to "the first person to demonstrate that C and E in the ACE model can act as legitimate variables in a causal model without the assumption that a specific, concrete environmental variable must correlate either 0 or 1 in siblings."
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Ah yes, but is it an unreasonable assumption when unpacked to the atomic level, where all events are binary? Either both twins get the event, or only one or the other does...
Now I wonder whether the guinea pigs in the first path diagram are legitimate or illegitimate?