Final answer:
The hypothesis that an audience can affect a performance by causing performance anxiety is the most reasonable account for the post-experimental statements by those in the neutral condition, according to the principles of the Yerkes-Dodson law.
Step-by-step explanation:
In Experiment 1, the hypothesis that would most reasonably account for the post-experimental statements made by those in the neutral condition is that an audience can affect a performance by causing performance anxiety. This hypothesis aligns with the idea presented by the Yerkes-Dodson law, which states that moderate arousal is generally best for performance, and too much or too little arousal can impair performance. The neutral condition in the experiment may reflect a baseline where performance anxiety is not influenced by the audience. Considering the details provided, such as an audience's presence reducing performance anxiety or a performer's anxiety not affecting their performance, these seem less direct in accounting for the statements from the neutral condition compared to the hypothesis about an audience causing performance anxiety.