Passage Summary: Scientists believed parents sound different when singing to babies because of their emotions. They proved this by showing that experts could tell the difference between 'baby-present' and 'baby-absent' songs just by listening.

Conclusion: The researchers' hypothesis that parental emotion noticeably alters the sound of their singing is correct.

Reasoning: In a controlled experiment, psychologists who did not see the recordings were able to correctly identify when a parent was singing to an infant 80 percent of the time.

Analysis: To strengthen this argument, we must bridge the gap between 'the songs sound different' and 'the difference is caused by emotion.' Currently, a skeptic could argue that parents might simply sing louder or slower when a baby is present for non-emotional reasons. Look for an answer that eliminates these alternative explanations or confirms that the parents' emotional state was the primary variable affecting the audio. Strengthening the link between the observed data and the hypothesized cause is the most effective way to support the conclusion.