Just as the presence of a reward can suggest that an activity is undesirable, the presence of a threat can suggest the opposite. that's why threats sometimes promote the very behaviors they are meant to discourage. for example, people who are motivated become motivated with the presence of a threat.

"For example, people who are extrinsically motivated become intrinsically motivated with the presence of a threat."

Explanation:

Extrinsic motivation is driven by a reward or an external circumstance. Intrinsic motivation is done for the enjoyment of the task itself.

What the paragraph implies is that a threat could suggest that an activity is fun or exciting, and thus many people may angage in it despite the possible negative consequences -and they genuinely enjoy it!

It's an example of inverse psychology, the "forbidden fruit effect".  

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