In the study cited in Johan's article, what variable is manipulated?

In research, the variable that is manipulated is often referred to as the independent variable. In Johan's article, if the focus is on how different levels of stress are applied to participants, then the amount of stress becomes the independent variable. This is typically manipulated to observe its effects on other outcomes, such as performance or decision-making. By varying the levels of stress, the researchers can examine how this variable influences behavior or cognitive processes in the participants.

In this context, the other variables mentioned, such as gender, task variety, and task duration, may not be the primary focus of manipulation if the study specifically aims to explore the impact of stress on participants. Therefore, the correct answer underscores the core aspect of the experimental design where the researchers are actively altering the level of stress to investigate its effects.