Passage Summary: Wirth claims that because a single 'manic-depression gene' hasn't been found, the idea of a genetic link is a myth; Chang argues that Wirth is ignoring the possibility that many genes could work together to cause the same effect.

Conclusion: The assertion that people are genetically predisposed to manic-depression is false.

Reasoning: The failure of researchers to identify a single gene responsible for the condition implies that no such genetic link exists.

Analysis: Wirth's argument suffers from a classic 'error of composition' or a failure to consider alternative causes. Wirth assumes that if a single, specific cause (one gene) is missing, the phenomenon (genetic predisposition) cannot occur. Chang’s criticism highlights that Wirth is overlooking the possibility of a more complex biological mechanism. When looking for the flaw, focus on the gap between the absence of a single factor and the total dismissal of the phenomenon itself. The correct answer should describe Wirth's failure to consider that multiple genes could collectively produce the predisposition.