The arguments for a strong role for epigenetics in same-sex attraction presented in the paper, are both concerned with twin studies. Firstly, they argue that there can be many epigenetically influenced differences in identical twins e.g. they find epigenetic effects on 600 genes that can create a difference of 200% between 2 identical twins, e.g., one twin might have twice as much of a particular protein as the other. But 600 genes out of a total of about 23,000 genes is only about 2.6%, not a large effect for something they claim has a dominant role in sexual differentiation. This small fraction is consonant with other work already known (Fraga et al., 2005)), unless the authors want to later make a special argument that these 600 genes are overwhelmingly critical in gender differences. So this is not a strong argument.