Michael,

I think you should read this great book on heritability:

https://www.goodreads.com/book/show/2105959.Making_Sense_of_Heritability

I've read more than 40 books this year (nonfiction) and this was the best one so far.

Besides, the usual criticisms of twin studies are now known to be wrong for certain. Why? Because modern genomic studies confirm the results of pedigree/familial studies without assumptions.

See:

Visscher, Peter M., et al. "Assumption-free estimation of heritability from genome-wide identity-by-descent sharing between full siblings." PLoS genetics 2.3 (2006): e41.

Davies, Gail, et al. "Genome-wide association studies establish that human intelligence is highly heritable and polygenic." Molecular psychiatry 16.10 (2011): 996-1005.