Weakening the Psychologist's Argument

In a recent study on the effects of caffeine consumption, researchers found that individuals who consume high levels of caffeine tend to report higher levels of anxiety. Based on this finding, the researchers concluded that caffeine consumption is a leading cause of anxiety.

However, a psychologist criticized the study, arguing that it fails to consider other factors that may contribute to anxiety levels, such as genetics, stress, or lifestyle choices. Furthermore, the psychologist suggested that instead of caffeine causing anxiety, it might be that anxious individuals are more likely to consume caffeine as a coping mechanism.

Which of the following, if true, would most seriously weaken the psychologist's argument?