Some experts claim that drinking coffee in moderation can improve cognitive function. They cite studies showing that participants who consumed a moderate amount of caffeine performed better on memory tests compared to those who did not consume caffeine at all.

However, other researchers point out that excessive caffeine intake can lead to anxiety and restlessness, which may ultimately impair cognitive performance. Therefore, the experts conclude that moderate coffee consumption is beneficial for cognitive function.

Which of the following, if true, would most seriously weaken the experts' conclusion?