Analyzing Causal Relationships in Caffeine Consumption

In a recent study, researchers found that individuals who consume high amounts of caffeine tend to have lower levels of anxiety than those who consume less caffeine. The researchers concluded that caffeine consumption reduces anxiety.

However, a follow-up study showed that individuals with lower anxiety levels are more likely to consume caffeine regularly. This leads to the suggestion that the relationship between caffeine consumption and anxiety may not be causal but rather correlational or reversed.

Which of the following most accurately describes the reasoning technique used in the researchers' argument?