Wirth presupposes that there’s only one possible cause of genetic predisposition— a “manic-depression gene.” But Chang points out that other factors, like multiple genes interacting, could cause someone to be genetically predisposed to manic-depression instead.

Wirth’s pieces of evidence— efforts that have failed to find a “manic-depression gene” and researchers who agree that no such gene exists— do not contradict each other. Also, Chang doesn’t criticize Wirth on this front; instead, he explicitly accepts Wirth’s evidence.

Presumably the researchers that Wirth cites aren’t experts in some field other than genetics. But even if they were, Chang doesn’t criticize Wirth’s evidence at all; he accepts it.

Chang presents evidence that disconfirms Wirth’s conclusion, but Wirth never claims to disallow such evidence on principle. And even if he did, that doesn’t describe Chang’s criticism of Wirth’s argument.

Wirth does treat the existence of a “manic-depression gene” as impossible, but so does Chang and so do the researchers. In other words, it’s not “merely unlikely” that a “manic-depression gene” exists, it actually is impossible. Also, Chang doesn’t criticize him on this front.